Reiji Furuhashi, a mathematics professor with a doctorate, met his fellow mathematics doctorate-holding wife Shizuru during university. They share one daughter, Fumino Furuhashi. His appearance features unkempt black hair, dull eyes with pronounced eyebags, and persistent stubble—marks of his academic obsession.

After Shizuru’s death, Reiji withdraws emotionally from Fumino, relegating communication to household notes. He rigidly champions innate talent over effort, dismissing Fumino’s astronomy dreams due to her weak math skills. He brands her pursuit "childish" and threatens expulsion if she persists. In one heated clash, he slaps her when she references her mother’s mathematical prowess—an act he instantly regrets but cannot confront.

His professional drive surfaces when he aggressively recruits math prodigy Rizu Ogata during an Ichinose Academy parent-teacher meeting. This provokes Fumino’s temporary departure. A turning point comes via Shizuru’s posthumous video message, revealing she faked her early math proficiency to impress him. Mediated by Nariyuki Yuiga, this shatters Reiji’s worldview. He reconciles with Fumino, apologizes for his past behavior, and endorses her astronomy path.

Post-reconciliation, Reiji displays wary acceptance of Nariyuki’s support for Fumino while scrutinizing their relationship. Privately to Hanae Yuiga (Nariyuki’s mother), he voices lingering guilt over mistreating his daughter.