Tsuyoshi Ohki (originally Tsuyoshi Sasaki following his parents' divorce) stands as Akito Hayama's closest friend. Parental separation shapes his family life, compelling him into a nurturing role for his younger sister, Aono. Despite these pressures, he retains a fundamentally gentle and kind nature.

He exhibits consistent cheerfulness, honesty, and sensitivity, contrasting sharply with Akito's abrasive personality. Tsuyoshi perceives inherent goodness in Akito and frequently defends him. However, this kindness coexists with a volatile temper; insults targeting his family, especially his mother, provoke uncontrollable, violent outbursts. Only Akito can calm him during these episodes.

Early in the narrative, Tsuyoshi develops a fleeting romantic interest in Sana Kurata, sparking temporary rivalry with Akito. This conflict resolves when Akito helps initiate Tsuyoshi's relationship with their classmate, Aya Sugita. Their bond deepens into a stable, affectionate long-term partnership that endures throughout the series. Tsuyoshi actively supports the central romance between Sana and Akito, directly informing Sana of Akito's feelings—though she dismisses this until Akito confirms it himself.

After his parents' divorce, Tsuyoshi moves with his mother and sister, an event where Sana and Akito lend assistance. This move reinforces his responsibilities as the older sibling. Years later, in the spin-off *Deep Clear*, he appears at age 26, maintaining his connection to Aya and their friend Fuka Matsui, underscoring the lasting nature of these relationships.