While his coat seems simple, its stiffly structured decorations show individualism.
Kayneth is a very wealthy and successful nobleman, a renowned magus from the Mage's Association, and a Euryphis lecturer in the department of Spiritual Evocation. Kayneth demonstrated genius level talent during his time as a student. He was the youngest lecturer to grace the podium of the Department of Evocation in its history. He has a ancient magical lineage that goes back for over nine generations. He was born with immense talent and rarely, if ever, encountered any hurdles in life. Due to his great lineage, he had extensive connections. He was expected to form the greatest faction in the Clock Tower. Waver Velvet is one of his students, but Kayneth cares little for him, especially after reading Waver's radical thesis concerning increasing magical potential. Kayneth decided to tear up his student's manuscript after publicly humiliating him during a lecture. Waver, in retaliation, stole Kayneth's artifact and entered the Fourth Holy Grail War. Kayneth originally intended to summon Iskander, but with his artifact, a piece of the torn mantle of Alexander, stolen by Waver, he instead summoned the Lancer-class Servant, Diarmuid Ua Duibhne.
He is engaged to Sola-Ui Nuada-Re Sophia-Ri, the daughter of the head of the spiritual evocation division, through an arranged marriage. They apparently do not get along and Sola-Ui considers Kayneth a coward for not going and attacking the opposing Masters face to face. While he loves her, she does not appear to share the same sentiments.
Kayneth is the epitome of how someone who believes themselves to be entitled at birth will never be happy. He is a cruel man with a selfish and arrogant personality. He holds a great amount of hatred towards anyone who wrongs him, wishing to torture and kill them if at all possible. He holds a especially great disdain for Waver's impunity in stealing his artifact and entering the War. Kayneth's reason for participating in the Holy Grail War is much like that of his student. He did not participate in the Holy Grail War due to necessity or duress, it was merely to add a splash of "martial prowess" to his life's story. That was it. Although he did not severely underestimate the Three Founding Houses, he did boast that Lord El-Melloi could overwhelm them by skills alone. The reason Kayneth brought many dangerous Mystic Codes had nothing to do with research, but for entertainment value. He desires to attain fame by fighting other Masters of great skill, talent and caliber, and the legendary heroes of the past that have been forth. He is more cautious compared to those like Waver Velvet and Irisviel von Einzbern, who both openly stand on the battlefield of the Servants. He prefers to hide himself during the fighting, and while technically practical, the action earns the scorn of both Rider and Sola-Ui.
He holds that all participants of the War will wish to fight honorably as magi, so he feels perfectly safe while inside his stronghold in the Fuyuki Hyatt Hotel. The existence of Kiritsugu Emiya, which goes against such principals to the point where he completely destroys the building rather than facing him in a battle of their magical prowess, greatly angers him. Upon the destruction of his Magic Circuits at Kiritsugu's hands, making it so that he can never utilize magecraft again, his goal shifts to claiming the Holy Grail to repair his Magic Circuits and regain his magecraft.
Kayneth cares for, and has genuine feelings for Sola-Ui, despite her loathing and disdain towards him. He even still displays love for her even after she blackmails and betrays him for his Servant Lancer.
Kayneth even considered the Heroic Spirits or Servants, to be part of his stock of Mystic Code. Because he never acknowledged the humanity of Servants, he did not understand Diarmuid's loyalty even until the very end. As a result, he effectively lost through self-destruction. If Diarmuid really appreciated Kayneth's talents and experience, then Kayneth would have treated him with a much softer attitude.
Takashi Takeuchi thinks he is a very human character because of the ease with which he could design Kayneth and draw his expressions as he breaks down. Gen Urobuchi wished to show his intellectual side more, and as Kayneth's role was only to be defeated, Urobuchi feels a strange affection for him.
Atsushi Ikariya was in charge of the character design for the anime version.
Kayneth is the Master of Lancer in the Fourth Holy Grail War and the most active master at the beginning of the War. His participation in the war was merely to increase his own prominence within the Mage's Association. His base of operations at the Fuyuki Hyatt Hotel is on the hotel's thirty-second floor and protected by twenty-four layers of bounded barriers, a countless amount of summoned monsters, various traps, and mystical spatial alterations in the lobby.
While Lancer and Saber fought, he hid behind his invisibility spell and observed the battle. Thanks to Lancer's clever tactic, the battle was turning out to be in his favor until Rider's interruption. When he found out that Waver's participate in the War as well, he revealed his identity to his former pupil and made a deadly threat. Soon after the three-way battle transformed into four then five-way with the sudden introduction of Archer and Berserker. In the middle of chaos when Berserker attacked Saber, Kayneth ordered Lancer to attack her as well. He initially refused but Kayneth forced him with a Command Spell. Thanks to Rider siding with Saber against Lancer and Berserker, and Kiritsugu finding Kayneth via infrared binoculars, he had to retreat.
After Lancer's battle with Saber, Kayneth criticized his Servant for not attacking Saber with all his strength. He winds up arguing with Sola-Ui who calls him a coward for hiding himself from the other Masters and criticizes him for his poor strategy and not thinking of having Lancer work with Saber to finish off Berserker. Their argument ended with Lancer standing up to his master. Lancer convinces her to stop arguing with Kayneth, her immediate submission to the Servant leads Kayneth to suspect Sola-Ui is developing affections for Lancer and would became a source of strife as Sola-Ui developed feelings for the Irish hero. Before he can address the issue, the lower levels of the hotel are attacked by Kiritsugu Emiya and Maiya Hisau. While Kayneth and Sola-Ui interpret the attack as one of the Masters driving people out of the hotel in order to attack Kayneth without any civilian casualties getting in the way and anticipated how to counter accordingly, Kiritsugu and Maiya had actually aimed to destroy the hotel's foundation. Kayneth, Sola-Ui, and Lancer all managed to survive the attack. Kayneth, who made his base the Hyatt Hotel, booked the top 24 floors and transformed it into a magical fortress with various magic barriers, traps, summoned creatures. But he didn't expect that someone would just choose to blow up the basement to collapse the entire building. Kiritsugu did exactly that with several C4 charges. With the protection of Volumen Hydrargyrum, Kayneth, Sola-Ui and Lancer just barely managed to survive the building's collapse. In one night he lost all the special equipment and Mystic Codes (aside from Volumen Hydrargyrum) he prepared for the War alongside his base.He moved to an abandoned warehouse and made it his new base. His relation to his fiance worsened with every failure. In revenge Kayneth assaulted the Einzberns. In the first battle with Caster, which was before his defeat, while Saber and Lancer held off Caster, Kayneth set out to defeat Kiritsugu. Using his Mystic Code "Volumen Hydrargyrum" that manipulates mercury, he sent it out to search for Kiritsugu. As Kiritsugu observed the Mystic Code's abilities he sees a sliver of mercury "look" through the keyhole of his room. Kayneth then sends his mercury to attack Kiritsugu, the whips of mercury slash through the roof and attack the Magus Killer. Using his internal Reality Marble of Time Manipulation, Kiritsugu escapes and after which he proceeds to hunt Kayneth. In his ignorance, Kayneth mocks Kiritsugu for his wielding of "useless" modern weaponry. After using a gun to set the mercury's defense to a certain degree, he then used a more powerful gun that managed to pierce the mercury's old defense, managing to hit Kayneth in the shoulder. Kayneth "corners" Kiritsugu and believes his opponent is going to use the same tactic and prepares to block another attack by maximizing his mercury's defense. Kiritsugu, using his Mystic Code pistol, then shoots his special Origin bullet at his opponent, causing Kayneth's Magic Circuits to go berserk when he blocked it with "Volumen Hydrargyrum" at full power. The bullet's destructive special effect depends on how many Magic Circuits are currently active in the target, and unfortunately for Kayneth, all of his were active for maximum defense. As a result, all of his organs were critically injured and his nerves were damaged beyond hope. Kayneth was subsequently rescued by Lancer right before he was about to be killed.
When he regains consciousness, Kayneth finds himself strapped to a bed. Sola-Ui then comes to tell Kayneth that Kiritsugu had destroyed all of his Magic Circuits and that he can no longer use magecraft ever again. Kayneth weeps in despair upon discovering that all of his Magic Circuits were destroyed and that his life and career as a mage is over. Sola-Ui pretends to sympathise and feel sad for him. She then suggests to continue in the war, as the Holy Grail could help in repairing Kayneth's Magic Circuits, and requests that Kayneth to hand over his command seals to her, so that she would take his place as Master. But Kayneth realises that Sola-Ui has gone nearly insane from her love for Lancer. He tries to convince her that it was to dangerous and they should get help first, but Sola-Ui then abused the situation to blackmail him, and proceeds to break off Kayneth's fingers one by one, forcing him to also give her his Command Spells. He eventually gives her his command seals, allowing her to become the master of Lancer. As a mage, his career is ruined. He was a cripple that could not even use magic anymore. The loneliness, frustration and jealousy almost drove him mad. He called the famous puppet-maker in secret. With first-class magical prostheses, Kayneth could finally move once again.
After the battle to defeat Caster, Kayneth approaches Risei Kotomine to obtain the one Command Spell promised as a prize to the Master who manages to defeat Caster. After forcing the reluctant Risei to comply based on Kayneth's assertion of Lancer's role in defeating Caster, Kayneth shoots Risei to prevent the regulator of the war from giving out any more Command Spells and to implicate Kiritsugu as the murderer. He discovers later that Sola-Ui was abducted during the battle against Caster.
Though furious at Lancer, who was also unaware of Sola-Ui's capture, they are forced to fight against Saber and Irisviel von Einzbern. Watching the duel from a hidden location, Kiritsugu appears before Kayneth with Sola-Ui at gunpoint and offers a mage's contract in the form of a Self-Geis Scroll, an absolute agreement which will render a mage permanently unable to perform sorcery if broken. This means that if Kiritsugu, as the target of the contract's curse, ever breaks the agreement, Kiritsugu will be unable to use sorcery ever again, which helps convince Kayneth to agree. Kiritsugu offers in the contract that Kayneth and Sola-Ui will not be harmed by him should Kayneth force Lancer to kill himself. After examining the contract to determine its authenticity and considering leaving the country with Sola-Ui alive, Kayneth agrees and orders Lancer to kill himself.
Lancer couldn't do anything but curse his opponents as he was forced to pierce his own heart by order of his Master's Command Spell. Guilt stricken and in despair, Kayneth asked Kiritsugu if he and his wife-to-be were free to go. Kiritsugu assures them that he will not harm them, giving him a brief moment of relief. But the contract had a loophole - Kiritsugu only swore not to kill them personally. It said nothing about his assistant, Maiya or anyone else in association with him harming them. From a distance, Maiya snipes both Kayneth and Sola-Ui, gravely injuring the mage and instantly killing his wife-to-be. The dying Kayneth who realized that the Magus Killer never intended to spare him or his wife, again pleaded for a swift death, but Kiritsugu refused due to the contract. Seeing this, Saber finally puts Kayneth out of his misery and kills him, decapitating him in the original and simply killing him in the anime adaptation, disgusted with her own Master's dishonorable methods.
Because of Kayneth's sudden death, most of his valuable research accumulated in the Clock Tower was unorganized and left unattended. Although they were almost lost, Waver eventually compiled a tome named "Lord Kayneth's Encyclopedia of Arcane Secrets", which laid the foundation for the flourishing of House El-Melloi and its descendants.
Kayneth is renowned for his prowess as a magi, known as the greatest prodigy of Eulyphis, a first-rate lecturer, and the most noted magi of the Association as of the Fourth Holy Grail War. His skills were not only in research, but also politics. With both the nine generation Archibald Magic Crest and his acknowledged talent, the inheritance of the prestigious El-Melloi title was a given to him. Kiritsugu Emiya initially regards him as one of the three most threatening magi involved in the war in terms of magical power and control, ranking second ahead of Tokiomi Tohsaka and behind Kirei Kotomine. He is one of the few magi Kiritsugu calls a "formidable enemy", whose strength is a great enough threat to make him utilize his Thompson Contender. Compared to a novice like Waver Velvet, who could be said to have "5 MP" in RPG terms, Kayneth would have "3,000 MP."
After the destruction of his Magic Circuits due to Kiritsugu's Origin Bullet, he becomes incapable of performing magecraft again. Simply surviving in his state is only due to receiving quick attention from Sola-Ui, and while she restores his internal organs, his muscles and nerves are irrevocably damaged. He is initially left completely paralyzed, and he cannot hope to be able to walk again. It takes a great sum of money and a puppet master to return his hands to working order, and that only allows him to move within the confines of a wheelchair. He still has the right to be Lancer's Master because the Command Spells are a different system of magecraft than Magic Circuits, but Sola-Ui forces him to allow her to have them.
Kayneth possesses the dual Elemental Affinity of water and wind, considered rare even among magi. He is an expert in Summoning, Alchemy, and Spiritual evocation, displays the ability to create a Bounded Field to keep regular humans from entering the battlefield of Servants, and can hide his presence and voice through either illusions or other presence-concealing magecraft. Though his position cannot be determined with human eyes and the location and gender of his voice are concealed by an unnatural echo, he is not hidden from a heat scope. He is proficient in healing magecraft, allowing him to quickly attend to Lancer's wound without being close or leaving any trace. He was really a man of many talents like Hiraga Gennai or Cao Cao.
While only the members of the three founding families normally have any influence over the system of the Holy Grail War, Kayneth displays his abilities in adding his own designs to the contract system to alter the cause and effect line between Servant and Master. He was able to overwrite the original rules with a secret technique that allowed him to separate the right of the Command Spells and the right of supplying prana to the Servant between two separate summoners. Along with Sola-Ui, they are a "two-person team" Master, with Kayneth taking control of the Command Spells and Sola-Ui providing the energy for Lancer. This allows Kayneth to have a great advantage in battle over Masters who must fight while also supplying energy for their Servant.
He creates a magic atelier within the Fuyuki City Hyatt Hotel's top floor, fortified with a twenty-four layer Bounded Field. Much like a "castle of magecraft", he supplies it with three magical furnaces for his personal use and numerous Mystic Codes, some ten-odd evil spirits and apparitions acting as guard dogs, various traps, and parts of the hallways that open into other worlds. While such an obstacle would be taken as a challenge for normal magi, Kiritsugu demolishes it in its entirety, and even the strength of the Bounded Fields would be unable to save the occupants from an one hundred fifty meter fall with the debris. He is saved by Volumen Hydrargyrum and loses all his wares, relegating him to an abandoned factory afterward.
Volumen Hydrargyrum: Marrow of the Moon Spirit (月霊髄液ヴォールメン・ハイドラグラム, Getsurei Zuieki?) is a unique Mystic Code created from mercury filled with magecraft. Kayneth excels in the art of manipulating flow, and the ability is shared between his rare dual elemental attributes. The shapeless mercury can be sculpted into any shape and controlled at will. It is his strongest trump card out of all the Mystic Codes he brought from England, and it always remains at his side as that in which he takes the most pride. Despite the loss of his wares in the destruction of the Hyatt Hotel, having Volumen Hydrargyrum makes him feel no overall loss in his battle strength. Uniting both attack and defense, it is a "perfect weapon" that acts as both his sword and shield.
He is able to easily transport it despite its normal weight being close to 140 kg with a spell of weight reduction, normally keeping it in a large porcelain vase in the novels and a small test tube in the anime. It takes on its true weight upon leaving his hand, and it is activated with the spell Fervor, mei sanguis. (沸き立て、我が血潮, Boil, my blood.?). Ten liters of mercury oozes from the mouth of the vase as if a disciplined primeval creature, or congeals on the ground from the test tube, becoming a large ball showing the mirror-like metal sheen of the substance. It begins to follow his commands upon forming, with a myriad of different applications. His initial command, Automatoportum defensio：Automatoportum quaerere：Dilectus incrisio (自律防御：自動索敵：指定攻撃, Automated Defense:Automated Search:Attack on command?), causes it to start following him and defend automatically.
Made to freely interchange between forms, it can be called the "epitome of the magecraft of fluidics" controlled with Kayneth's immaculate skill to perfection. Its regular form is a large ball three meters in diameter that follows and protects Kayneth. It responds to his movements and commands, shifting between forms for different tasks. It can take on a shape like a jellyfish with a flat platform for Kayneth to ride, and it can uses its tentacles to climb, attack, and defend with ease. Once he steps off, it quickly reverts to its ball form. Upon losing its prana source, it reverts to being only a puddle of mercury. Upon being passed onto Waver Velvet, he modifies it into a Maid Golem.
Mercury is a heavy liquid in room temperature, and it posses great kinetic energy when moving rapidly under high pressure. It can be manipulated into shapes like whips, spears, and blades at will, and its sharpness can even overwhelm laser rays. Nothing can be said to be unbreakable before it, allowing it to destroy the most solid defense. Compressing itself into a thin edge a few millimeters thick before striking, its blade rivals pressurized water jet cutters. The mercury blade can cut through any solid defense as if cutting through tofu, even should it be titanium alloy or diamond. His command of Scalp (斬, ?) against a barrier causes it to change into a long and thin ribbon in the shape of a whip that strikes out at the target with a whipping motion, and against a living target, two whips can strike in a pincer motion to cut at the target from both sides. It also acts according to his state of mind, such as randomly attacking everything in sight while he is agitated.
The weakness of the attacks is that controlling mercury with pressure means only parts with large mass can exert their full power. The whip shape has the greatest power at its base to control the extremely quick attacks, but the tip has no power behind it. It cuts with the power of centrifugal force, causing the attack to become weaker towards the tip. Though the attacks are so fast that only Innate Time Control allows Kiritsugu to initially dodge, being experienced in melee combat allows him to use that weakness to easily predict its path and dodge even without enhanced speed. It is also possible to detect its maximum range, its speed and power being most fatal within a seven-and-a-half meter radius and still potent up to just over ten meters, to keep from being a target at all.
Upon activating Volumen Hydrargyrum's automated defense mode, it instantly responds to anything that can threaten Kayneth and defends appropriately. While it is meant to be able to absorb Gandr shots, deflect magic swords, and break through supernatural fire, ice, and lightning, it can also protect against modern weaponry with ease. It creates a protective membrane when infused with his prana, with surface tension that is comparable to steel even when barely a millimeter thick. It can react in a split second to defend against magical flames and mist, gunfire, mines, and even the immediate destruction of the Hyatt Hotel and the fall from the top floor. Projectiles are reflected and rendered ineffective no matter if he is attacked from the front, surprised from behind, or surrounded on all sides.
Though the protection is potent, it is possible to get past it at least once by observing its nature. The speed of its defense comes from hydraulic pressure, allowing it to spread the membrane faster than a bullet. The limits of hydrodynamics make it impossible to return as quickly from the protective membrane to its blob form through only pressure, so, after adapting to the best physical shape to protect against the current threat, an immediate attack of greater destructive power can pierce it before it has time to react and form a more powerful defense. It is impossible to properly aim at Kayneth behind the wall, and the tactic only works once, as Kayneth will then keep the defensive power firm enough to always block the stronger attack with all his available prana.
There is another form of defense used against Kiritsugu, activated with Fervor, mei sanguis. (滾れ、我が血潮, Boil, my blood.?), that creates a large number of spikes in front of Kayneth that reach from the floor to the ceiling, resembling a bamboo forest. The membrane is useful against magical attacks, but bullets only need to be stopped from proceeding in a straight line to render them harmless. Creating the columns requires a much greater energy expenditure than the membrane, as each spike that has been twirled to be as thin as steel wire must be given the strength and malleability to deflect bullets. This action pushes him to the limit to create an impregnable fortress that easily causes bullets to ricochet within the gaps of the spikes while not allowing them to reach Kayneth. Their defense against a stronger bullet like that of the Thompson Contender is much greater than the membrane, and they can quickly close around the bullet to become a single column that closes the attack off completely.
Its automated search function causes it to reach out with numerous tentacles of mercury around the area to search for targets. It takes its time to slowly create a web in order to seal off exits and search rooms. It can easily search the entire first floor and second floor of the large Einzbern Castle in a short amount of time, easily slipping its tentacles into spaces as small as keyholes to search rooms. Kayneth can then follow its trail to the target. Due to being far from the main body of mercury, they are not as agile in attacking as the main whipping attacks. The specific command Ire: Sanctio. (追跡抹殺, Track and kill?) brings another immediate search of the area.
Mercury has flaws as a sensory tool in that it is limited in its ability to communicate information back to Kayneth. He can control it easily due to lacking specialized senses such as sight, smell, and taste, so he cannot rely on it to be able to sense targets in that manner. He uses its extremely sensitive sense of touch to make judgements based on vibrations in the air, sense differences in air temperature, and locate heat sources. This makes it possible to fool if the target can alter their heart beat, breathing, and body temperature to a minimum as to make their existence transparent before it. It cannot differentiate between the normal sounds of the world and a human body in that state, so it does not process it as a human target.