PersonalityEditSouichirou is widely regarded as an austere and impersonal man. He is serious, sometimes emotionless and does whatever Caster says in battle even though he does not care about the Holy Grail.
Souichirou found that when he accomplished his first and only mission, he felt only emptiness rather than any other emotion. Hoping to find meaning in his life, he left the organization and became a school teacher. He found no meaning and found that he could acknowledge only his own existence. He described himself as a "decayed killer" (朽ち果てた殺人鬼, Kuchihateta satsujinki?). 
When he met Caster, he felt something move within him. While he had no interest in the coming Holy Grail War, Souichirou helped her just because she asked for it. His motive to fight is similar to Shirou Emiya..
According to Fate/hollow ataraxia, he finally realizes that he regretted his whole life and how he killed others, knowing and understanding that what he was doing was wrong yet still continuing that kind of life. Uncertain of how to make expiation for his guilt, he suffered during his whole life yet managed to find some peace with Caster.
BackgroundEditBefore he came to Fuyuki City, Souichirou had been raised from childhood by an organization that trained young children to become professional assassins. The children were raised while isolated from the rest of society in places like forests. They were taught skills required for killers and were treated as if they were tools. Teaching knowledge on living as normal humans was minimal to ensure the skills of the potential assassins in their care would remain adequate and efficient.
He left the organization and appeared in Fuyuki City, two years before the Fifth Holy Grail War. He chose to become a school teacher, which was to serve as a cover for his job as an assassin. He primarily lectures on World History, though he also holds classes in Ethics. Kuzuki is well respected among staff and students. He was well-liked by the chief priest at the Ryuudou Temple and was granted residency in the temple.
Kazuhiro Nakata voiced Souichirou in the Studio Deen version of Fate/stay night and Carnival Phantasm. Unshō Ishizuka voiced Souichirou in Fate/Unlimited Codes. Masaki Terasoma is the latest voice actor who voiced Souichirou in the Ufotable adaption of Fate/stay night.
FateEditSouichirou is not a mage, but a highly trained assassin. After Caster killed her previous master and summoned the servant Assassin, she chose Souichirou to be her new master. He has no interest in the Holy Grail war and acts mainly according to the will of Caster. It seemed that the two have a strong love interest in each other. It's implied that he sleeps with her on at least one occasion to restore her mana, as Shirou had done with Saber. His identity as a Master is not made known in the Fate route and his fate there is thus unknown.
Unlimited Blade WorksEdit
Souichirou is not a mage, but a highly trained assassin. After Caster killed her previous master and summoned the servant Assassin, she chose Souichirou to be her new master. He has no interest in the Holy Grail war and acts mainly according to the will of Caster. It seemed that the two have a strong love interest in each other. It's implied that he sleeps with her on at least one occasion to restore her mana, as Shirou had done with Saber. Kuzuki only plays a major role in the Unlimited Blade Works route, in which he is revealed as Caster's Master. After Caster is killed by Archer's hail of swords, he is defeated in melee by Archer. He dies after Caster does in the anime and the Unlimited Blade Works scenario. In the anime, he tells Caster that he is fine after she sacrifices herself to save him and dies. Little does she know, he dies moments after her, having lied to her so that Caster could die feeling relieved.
Souichirou is not a mage, but a highly trained assassin. After Caster killed her previous master and summoned the servant Assassin, she chose Souichirou to be her new master. He has no interest in the Holy Grail war and acts mainly according to the will of Caster. It seemed that the two have a strong love interest in each other. It's implied that he sleeps with her on at least one occasion to restore her mana, as Shirou had done with Saber. In the Heaven's Feel route, he is only mentioned in passing, when Saber confronts Caster at the temple. Shirou and Saber find her covered in blood with Rule Breaker out and coated as well, a man Shirou recognizes and believes to have been Caster's master, stabbed fatally and in a pool of blood on the floor, already a corpse. Caster does not seem to know she killed him, however.
In Fate/hollow Ataraxia, he lives with Caster and Assassin, with Caster attempting to fill the role of a loving housewife to him. In one of the loops, he fought and died against a servant. When Avenger makes his move, Souichirou fights the dog-shadows right beside his Caster and Assassin.
Kuzuki is the Master of Caster, but he is not a magus nor does he have Magic Circuits. He only acts as her anchor to the world, while she must subside off of the prana taken from the leylines and the town in order to upkeep her existence. His sperm was still capable of stabilizing her enough to establish their contract after she was brought within Ryuudou Temple’s Barrier.
PhysiqueEditKuzuki is normally physically around Shirou in strength according to Saber, and not knowing his fighting style, she believes he would do as well in a true battle as any ordinary man. With his fighting style, he is capable of dealing a blow to the temple that would crush a man's skull. While strong, such a blow is only enough to move a Servant's head slightly from the impact, so he could never hope to face them in battle normally. His fighting abilities can be called "superhuman", but fighting a Servant requires much more than that. He gains an abnormal strength from Caster's Reinforcement magecraft, so he is referred to as a "Saber-class defender."
His fists are strengthened to the point where they can be called steel. He is able to destroy Rin's head with a single blow, removing everything above her neck, and he is able to create a hole through Shirou's chest with a single strike. He can shatter Shirou's wooden sword, reinforced to where its can be considered a metal rod, with his fist. He can strike and deflect Shirou's projected Noble Phantasms, and attack with enough force to damage Shirou's arms just from defending against his attacks. While his fists don't have enough strength to destroy a Noble Phantasm, he is able to destroy Shirou's imperfect replications with pure force. He can catch a full powered slash from Invisible Air between his elbow and knee without suffering any damage, and he can strike Saber's armor directly with no effect to his hands.
While his fists are enhanced, he is still only a human with no Magic Resistance. The greatest threat to him isn't a Servant, but a magus who is able to attack from a distance. He is still able to easily close the distance to Rin before she can even realize it, strike the center of her body to render her unable to breath or use magecraft, and then decapitate her in an instant. He could also win against Caster as long as he was able to get the first strike, but she would easily obliterate him with spells if she were to strike first. The strengthening requires Caster's upkeep, so his strength immediately drops upon her death.
Kuzuki appears to be completely ordinary at first sight, but Saber does note that his breathing is too natural and too controlled. While she suspects nothing off about him, she says his breathing is ideal for a human and his steps have no wasted movements, and she admires and praises him as the greatest she has seen in that respect. While there are those who are naturally born with such talents that go on to become great magi, she feels that he acquired his breathing and walking techniques through training. He is not a magus or capable of replicating potent thaumaturgy due to his talent, but his everyday training has affected him in the perfect way that is admirable to Saber who could not attain it even through training.
He uses his normal appearance to gain an advantage over opponents who underestimate him. When ready to fight, he doesn't radiate any enmity or bloodlust, and simply enters his battle posture while giving off a clear murderous intent that hides his scariness. He is called more like an assassin than Assassin due to this. Kuzuki is able to surprise Caster by appearing before her without making the sound of footsteps or showing any sense of his presence. He is the only one to notice Shirou observing them at the church, and as he doesn't have any emotion towards enemies, he is able to quickly attack without hesitation. He is unable to notice Archer before he kills Caster because he cannot detect prana.
SnakeEditKuzuki uses a fighting style called Snake (蛇, ?) that he learned over a period of twenty years while being raised as an assassin. The crux of the fighting style is to keep his arms flexible like whips while quickly moving at unexpected right angles in order to keep it from having a concrete shape. He keeps his arms and stance steady like a rock while waiting to strike, and he is able quickly close in without revealing any hints as to how he advanced. His arms quickly extend, and his forearms move even more quickly from that point to strike with his "mountain-like fingers." The strikes come from the outside to the inside, the swinging arms changing direction using the elbow as the fulcrum to strike from impossible directions. He can easily strike the back of the head from a very close distance completely unbeknownst to the target. Even if that strike is seen through, he can easily alter the circular orbit of the path into a straight line directed from above to smash down on their head. He can quickly reach through an opponent's guard to strike them, and then use the "body of the snake", an elbow, to deal an additional blow. Each is accurately aimed at the target's vital points and weak spots, and under Caster's reinforcement, they can damage Saber bluntly through her armor to directly target the core of her body. They are heavy and rapid in their intensity, rendering them almost invisible to an unexpecting fighter, but they are not an instant kill to someone like Saber. They are dull and sharp, and contain a poison that will lead to death, allowing him to go for the kill if the opponent even drops their guard for a second. Dodging the attacks does little, as his fists are "live snakes" capable of changing path to come back to the target. Saber is damaged badly because she has the reflexes to dodge the strikes, thus she got repeatedly "bitten", differing from Shirou who cannot see the fists and does not posses the reflexes to dodge them. He can only desperately parry and receive glazing blows. Kuzuki can also use the style in defense, making him able to react to a perfect surprise attack, in charging speed, footwork, timing, and the attack itself, from Saber and catch a invisible sword horizontally slashing at him with only his elbow and knee. While techniques to stop an an enemy's blade with bare hands are practiced by experts in the present, the idea of a human doing such to a Servant is unbelievable to Saber. All of the strikes up until that point are performed with only his left hand, as he doesn't use his right hand until the final strike. Unlike previous attacks that follow a arcing line, he executes the attack as a point and thrusts his fist like a lance directly at his target. Using precise accuracy, the attack can pierce through a throat, break bones, and smash a head to pieces. The attack managed to slay Rider, ripping her neck apart in one blow as if someone used a jack to rip off the meat and bone by crushing it, and it is capable of destroying Shirou's Kanshou and Bakuya guarding his chest and still blow him five meters away into a wall from the impact. Even if the strike is dodged, he can instead dig his fingers into the target's neck, as they can easily forget that a hand is originally something used to grab rather than punch. He can lift Saber with just his hand from this position and throw her at a speed of two hundred kilometers per hour due to his enhanced strength, knocking her out of the fight for a time after she collides into a wall. He mentions that he underestimated her, and that should have applied one more attack afterward.
He is able to accurately adapt his attacks in order to defeat opponents. He focuses his attacks on Saber's body and arms due to her protective armor, all while waiting to launch a fatal strike at the head. He doesn't attempt to follow her while she retreats, but instead uses various blows to keep her from escaping his range to keep his advantage over the reach of her sword in close quarters. The blows aren't too heavy even though she cannot avoid them, but the pain from each blow numbs her mind. She becomes unable to counterattack from the heavy rain of attacks, and even her Instinct eventually fails her during the final moments of the battle. Kuzuki recognizes that Shirou relies on his Noble Phantasm to defend, so he makes it his goal to destroy the Projections in order to land a fatal blow.
The main idea of the style is to confuse the opponent and slay them before they can understand the mechanics of it, and it is further backed by his normal appearance that doesn't give away the idea of him being a trained killer. As an assassination technique only meant to be used once before the assassin kills themselves, it is something that must be fatal on the first strike. While the blows are strong, they are too odd and easily seen through if the opponent is not confused, showing the difference between a "technique" that is trained to an art and an "action" that is trained to the utmost limits. Catching Saber's blade and striking her in the back of the head unsettles her greatly due to her belief that he is just a normal human and her inability to understand the nature of such techniques. Due to experiencing the strange attacks for the first time, she is not able to understand the fighting style with her eyes, leading to her taking most of the attacks, and must rely on her honed Instinct simply to avoid the decisive blows.
While easier to comprehend on subsequent battles, it is still strong enough of a style that Shirou is barely able to hold him off for a minute at most even with Noble Phantasms and Archer's fighting style on his third battle with Kuzuki, as he cannot even begin to comprehend how to fight against the style. It would not work on Saber twice, so he retreats from her with haste once she is ready to fight again. While Shirou could never cope with it no matter how many strikes her receives, Saber is already used to them after their one exchange. Rin believes Archer would have no trouble with him due to having knowledge of it. Servants are more emphasized by their talent than their hand-to-hand prowess, so Bazett Fraga McRemitz, an expert at hand-to-hand combat thought of and developed by human beings, may be able to adapt to the style more quickly. He would likely win their first fight, very easily if Bazett underestimates him for a normal person, but she would be able to completely win against him in a second engagement. Out of himself, Bazett, and Kirei Kotomine, he would win as long as it is limited to one fight and he starts with a surprise attack.