Sōichirō Kuzuki teaches social studies at Homurahara Academy, focusing on World History and Ethics. He shares residence at Ryūdō Temple with his partner, Caster. Trained from childhood as a professional assassin by an organization that isolated him in environments like forests, his upbringing prioritized lethal skills over socialization, treating children as tools. After leaving and feeling emptiness post-mission, he moved to Fuyuki City two years before the Fifth Holy Grail War, adopting teaching as cover. Despite this past, he garners staff and student respect through contributions like enhancing school clubs and revising rules.

In this universe, Kuzuki maintains a stoic, expressionless demeanor, appearing impersonal and austere. His minimal interactions reflect his isolated upbringing. His relationship with Caster remains central; she actively pursues a domestic role as a "loving housewife," attempting to cook for him. Her dedication is evident when she requests cooking lessons from Shirou Emiya to prepare an autumn dish for Kuzuki. He accepts her efforts with characteristically reserved reactions.

Kuzuki appears sporadically in background roles, such as communal gatherings at the Emiya residence. His most notable involvement occurs in Episode 9, where Caster’s culinary ambition drives the plot. He also features in minor appearances during group meals or seasonal events. Outside this series, Carnival Phantasm depicts him receiving a good-luck charm from Caster, leading to unexpected rewards and culminating in his proposal to her. This lighter spin-off portrayal contrasts his main-series fate.

His assassin background informs physical capabilities like perfected breathing techniques and muscle efficiency, though unexplored in the culinary narrative. Emphasis rests instead on his quiet domestic life with Caster and his educator role.