Itsuki Tomofusa is introduced as the class president of Saya Kisaragi's second-year class in the fictional, seemingly peaceful village where much of the story takes place. He is a gentle and responsible student who wears round glasses, the most recognizable feature of his appearance alongside his neat school uniform. At the outset, his primary concern appears to be looking after his classmates, especially Saya, who often misses school for what she explains as her duties performing ancestral rites. He frequently lends her his notebooks and worries about her well-being, actions that stem from his sincere, if unrequited, romantic feelings for her. His efforts to get closer to Saya are portrayed as earnest but clumsy, often met with obliviousness on her part and playful teasing from other friends, cementing his early role as a kind-hearted and somewhat pitiable figure in the school setting.

However, this persona is eventually revealed to be a constructed role. Itsuki is not an ordinary student but one of the participants in a cruel, elaborate experiment orchestrated by Fumito Nanahara. Alongside his classmates, he was an actor hired to inhabit a fabricated identity and daily life, all designed to manipulate and observe the memory-altered Saya. His true purpose was to play the part of a caring friend to reinforce the illusion of her normal life, with his actions and words monitored as part of the experiment. Despite the artificial nature of their shared reality, Itsuki finds his feelings transforming from an act into something genuine; he confesses that, while his role was a fabrication, his love for the person he came to know as Saya became real.

Itsuki's role in the story shifts dramatically when the experiment begins to collapse. While almost all of his other classmates are killed in a staged attack by an Elder Bairn, Itsuki is one of the few survivors, an outcome that casts suspicion on his true nature. His genuine nature is ultimately proven in the final act. As Fumito attempts to escape and orders his soldiers to eliminate Saya, Itsuki chooses to betray the experiment and its architect. He selflessly intervenes, using his own body as a shield to protect Saya from a hail of gunfire. Mortally wounded, he dies in her arms. With his final breaths, he confesses that he fell in love with her, adding that even if the gentle, smiling Saya he knew was not entirely real, that part of her was still a part of her. His death is a pivotal moment, representing one of the few authentic acts of kindness and sacrifice Saya experiences amidst a world of deception and violence.

In terms of development, Itsuki’s journey is a tragic arc from a seemingly unassuming background character to a key conspirator, and finally to a genuine hero who defies his assigned role. His most significant relationships are with Saya, for whom his love becomes his defining motivation, and with Fumito Nanahara, the orchestrator he ultimately turns against. Regarding notable abilities, Itsuki possesses no supernatural powers or combat skills, distinguishing him from the series' hunters and monsters. His strengths are entirely human: his loyalty, his capacity for genuine emotion, and the moral courage to sacrifice his life for another. He is remembered as a rare good person within a deeply cynical narrative, one whose love transcended the artificial world he was forced to inhabit.