Hajime Seki is a supporting character in The Neighbor No. Thirteen, portrayed as a former middle school classmate of the protagonist Juzo Murasaki and the primary antagonist Toru Akai. In the present timeline of the story, Seki works alongside both men at a construction site, where Akai serves as a supervisor. Unlike the withdrawn and socially isolated Juzo, Seki is characterized as a friendly and approachable individual who makes an effort to be kind to Juzo despite his asocial tendencies, attempting to bridge the gap left by Juzo's traumatic past. This amiable nature sets him apart in the grim and hostile environment of the worksite, where Akai’s bullying behavior persists into their adult lives. Seki’s role in the narrative is primarily that of an observant colleague who grows increasingly suspicious of Juzo’s strange and erratic behavior as the story progresses. Concerned by these developments, he attempts to warn Akai, approaching him at the construction site to share his worries. However, Akai dismisses Seki without giving him a chance to speak, a fatal act of negligence. In that moment, Seki catches sight of the menacing figure known as Number 13, the brutal alternate personality born from Juzo's psychological trauma. His discovery proves to be his final act, as Number 13 immediately kills him. Through this sequence, Seki serves a crucial function in the plot: his death marks a point of no return, demonstrating the escalating danger of Number 13 and eliminating one of the few genuinely decent characters who could have intervened or provided support to Juzo. His fate underscores the story's central theme of how unchecked violence and past trauma can destroy not only the intended targets of revenge but also the innocent and well-meaning individuals caught in the crossfire.