Hiroto Honjou is a black-haired, green-eyed high school student who shares a class with Mamoru Suzuhara. His participation in the seven-day birthday event at the Satomi coal mine facility begins reluctantly after childhood friend Saki Akutsu deceives him. Having known each other since kindergarten, Saki lured him by claiming she had something important to say, later admitting this was a ruse and mocking him about expecting a love confession. Despite Hiroto's grumpy demeanor and complaints about being dragged along, Saki values their friendship and invited him to prevent his exclusion. She considers him more intellectually capable than Mamoru, labeling him a studious person with limited outside interests.

Throughout the event, Hiroto consistently voices concern over the consequences of the group's actions. He opposes Aya Chiyono's decision to hide from her father, warning it would provoke parental outrage. Later, he challenges the group's choice to shelter Malet, arguing that knowingly harboring someone involved in legal troubles constitutes criminal complicity. His cautious and pragmatic outlook contrasts with his peers' more impulsive decisions.

After public exposure of the students' identities, Hiroto faces severe personal repercussions. Online users uncover his secondary social media account, revealing past posts detailing intense familial strife. These include resentment toward his parents for their excessive focus on academic achievement and extreme sentiments wishing for their death. The disclosure triggers public criticism labeling him disturbed, with suggestions his introverted nature could lead to explosive behavior. His online critiques of the group also surface, describing Aya's plan as childish and singling out Saki's emphasis on "sandbox friendship" as particularly immature. Saki encounters these posts, causing her visible distress.

As events progress, Hiroto undergoes substantial development. He confesses to living a life dictated by his parents' expectations, a reality he claims to despise. Admitting his grumpy exterior served as a facade, he reveals genuine appreciation for participating in the seven-day experience. In a moment of vulnerability, he bonds with Souma Ogata, who faced bullying in middle school. Hiroto expresses disregard for Souma's past while anticipating future baseball games together. He further dismantles perceptions of his intellectual persona by disclosing that the books he reads are adventure novels, not academic texts.