Hitomi Kashiwa is a pivotal character from the story, primarily known as the high school senior or senpai of the protagonist, Tatsuhiro Satou. During their school years, Hitomi was the president of the literature club, which she convinced a reluctant Satou to join. Despite the club's intellectual premise, the two spent most of their time playing cards and indulging in lengthy discussions about elaborate conspiracy theories, a pastime that would have a lasting impact on the younger man. In her adult life, Hitomi becomes a civil servant, a position that provides financial stability and social standing but is also a source of immense stress and dissatisfaction.

Hitomi is characterized by a deeply ingrained sense of paranoia and depression. She is prone to viewing the world and its events through a lens of interconnected conspiracies, which she adopted as a coping mechanism years before Satou did. Her mental state is fragile, leading to insomnia, suicidal ideation, and a reliance on a variety of antidepressant and other psychotropic medications. While she holds a respectable job and eventually secures a stable relationship, she remains plagued by a sense of emotional emptiness and a feeling of being unable to connect with her seemingly successful life. Her motivations often stem from a desire to escape the crushing mundanity of her reality, seeking either the ultimate escape of death or transient, intense emotional experiences to feel alive.

Within the narrative, Hitomi serves as a significant, albeit complex, figure from Satou's past. She is the person who first introduced him to the conspiracy mindset that later becomes intertwined with his own hikikomori (social withdrawal) condition. Her reappearance in his life during his years as a recluse creates turmoil, as she represents a lost potential for a normal relationship and inadvertently reinforces his negative self-image. A key moment in their dynamic occurs when she invites Satou to a gathering that is, in reality, a group suicide pact. While she abandons the idea after receiving an unexpected marriage proposal from her boyfriend, Akira Jougasaki, the event pushes Satou to a critical emotional breaking point. Later, even after her marriage to Jougasaki, she propositions Satou for an affair, revealing her continued internal dissatisfaction and her view of him as someone with whom she can be her true, troubled self without pretense.

Hitomi's key relationships are fraught with complexity. Her connection with Satou is a blend of genuine, if distorted, affection and a manipulative dynamic where she uses him as a failsafe, a person whose life is worse than hers to make her own seem more bearable or as a vessel for her destructive impulses. She has known his innermost weaknesses longer than almost anyone else and shows a remarkable tolerance for his erratic behavior. Her relationship with her husband, Akira Jougasaki, is a central part of her adult life. Although he represents conventional success and stability, their marriage is not entirely fulfilling for her, as she confesses to feeling neglected and is tempted to stray. In some versions of the story, her psychological instability manifests in volatile and even violent behavior towards her laid-back husband.

Throughout the story, Hitomi demonstrates little change in her fundamental nature. Her life follows a trajectory from a troubled high school student to a materially successful but emotionally unwell adult, and finally to a married woman and mother. This progression is less about personal growth and more about conforming to societal expectations while her internal struggles continue. Her main notable ability lies in psychological insight; she is perceptive about Satou's nature and knows how to draw him back into her orbit, for better or worse. While she ultimately finds a form of conventional happiness in her family life, her presence throughout the story serves as a haunting reminder of the past and the persistent, corrosive nature of deep-seated paranoia and depression.