Kuzuki is the homeroom instructor of Rin's class.
Before arriving in Fuyuki, Kuzuki was raised since childhood by an organization that trained children into becoming professional assassins. Children raised by this group were isolated from the outside world and only taught the necessary skills considered adequate for assassination. He left the organization years later and arrived in Fuyuki two years before the start of the 5th Holy Grail where he became a high school teacher teaching World History and Ethics. Kuzuki resides at the Ryuudou Temple, where he is respected by the chief priest of the temple and the chief's son Issei.
Kuzuki was created as a major character for the Fate/Stay Night visual novel game created by Type-Moon. He has appeared in anime and manga adaptations of the franchise consisting of Studio Deen's 2006 TV anime adaptation and the 2010 movie adaptation of Unlimited Blade Works, a manga adaptation of Stay Night, Carnival Phantasm and Ufotable's TV adaptation of Unlimited Blade Works.
Kuzuki is voiced by Patrick Seitz in English anime versions of Kuzuki. His voice in Japanese had been done by different voice actors depending on the medium. Kazuhiro Nakata voiced him for Studio Deen's adaptations of Fate/Stay Night and Carnival Phantasm. Unsho Ishizuka voiced Kuzuki for the Fate/Unlimited Codes video game. Masaki Terasoma voices him for Ufotable's UBW TV adaptation and the Playstation Vita version of Fate/Hollow Ataraxia.
Kuzuki stands at 180 centimeters tall (an even 6 feet) with brown-greyish hair and eyes. He is normally seen wearing glasses and is dressed in formal clothes when appearing in front of others.
Kuzuki is a man who displays little emotion and is always serious in the course of his actions. He left the world of assassination to find meaning in his life after realizing how empty he felt from taking part in it. He still felt empty with his new life as a teacher, considering himself a "decayed killer" as he could only acknowledge himself over others. However upon encountering Caster, Kuzuki felt something move in him and he felt compelled to act on her behalf, regardless of the casualties that piled up from her actions within the city.
Kuzuki first came upon Caster struggling up the steps to the Ryuudou Temple as she was on the verge of disappearing. Begging him to help her, Kuzuki agreed to Caster's plea as he agreed to a new contract with her and helped her recover. The two developed a strong romantic interest in each other and Kuzuki was able to talk the owners of the temple to let her stay there under the guise of Caster being his fiancee.
He has little role in the Fate and Heaven's Feel routes of the series, as his fate is unclear within the Fate route and was seen to have been slaughtered beyond recognition by Caster in Heaven's Feel when her body was being used as a puppet by one of Zouken Matou's worm familiars.
Kuzuki's role is a major one in Unlimited Blade Works as Caster uses Reinforcement magic on him to serve as her enforcer against Shiro, Rin and Saber. He winds up being killed by Archer in melee combat after Caster is slain by a volley of swords coming from the Servant.
Kuzuki lacks any abilities in magic, as having intercourse with Caster was only enough to allow him to establish a contract with her and keep her from disappearing. His assassination training gave him heightened strength and speed that surpass a normal human as such where he would be capable of crushing a man's skull with a blow to the head. However, his abilities are inadequate for fighting against a Servant unless he's augmented via Reinforcement magic from Caster. With Reinforcement, he is capable of fighting on the level of a Servant as his strength and speed are greatly enhanced while the spell is in effect.
Kuzuki is trained in a fighting style called Snake which is focused around delivering blows in a quick, snake-like matter at various angles to force his foe on the defensive and attack vital areas of his foe. His limbs are rather flexible as he is capable of altering the trajectory of his blows while in motion to strike another part of his opponent's body in the event they are able to anticipate his attacks. The fighting style's purpose is to confuse a foe and kill them quickly before they realize the mechanics behind it. However, skilled fighters such as Servants would be able to understand how to counteract the Snake style if they are able to engage Kuzuki in a second battle with him.