Sōichi Tatsumi is a first-year doctorate student in agricultural science at N University in Nagoya. As the eldest child of entomologist Soujin Tatsumi, he assumed responsibility for his younger siblings, Tomoe and Kanako, following their mother's early death, though household duties primarily fell to them and an aunt. This role fostered overprotectiveness, particularly concerning Tomoe's romantic life.

A defining trauma occurred during university when a male professor attempted to sexually assault him under false pretenses in a laboratory. This incident cemented intense homophobia, leading him to view homosexual men as "perverts or Satan's rape demons." His reaction extended to vehement opposition to Tomoe's relationship and marriage with Mitsugu Kurokawa, including attempts to interfere. The trauma also motivated him to study kenpo for self-defense.

Personality-wise, Sōichi exhibits extreme anti-social behavior, preferring laboratory work over social interaction. He is hot-tempered, verbally profane, and physically violent—often kicking or shouting at his research assistant, Tetsuhiro Morinaga—earning him a "tyrant" reputation. He struggles with self-awareness, particularly regarding his sexuality and emotions, frequently denying or rationalizing his feelings. Despite his aggression, he displays shyness and embarrassment when confronted with personal vulnerability.

His contentious relationship with Morinaga, who is openly gay and in love with him, is central to his development. Aware of Morinaga's feelings, Sōichi permitted their professional relationship under strict boundaries. A pivotal moment occurred when Sōichi accidentally consumed an aphrodisiac at Morinaga's apartment, leading to a non-consensual sexual encounter. Though initially furious and betrayed, Sōichi later experienced severe anxiety when Morinaga disappeared, realizing his emotional dependence. He sought Morinaga's family contacts and frequented his apartment, acknowledging the loss as "the worst he ever felt." Upon Morinaga's return, Sōichi reluctantly agreed to continue sexual relations to maintain their connection, rationalizing it as transactional while suppressing deeper attachment.

Throughout their relationship, Sōichi grapples with internal conflict. He exhibits jealousy and possessiveness, reacting negatively to others' interest in Morinaga, yet denies romantic feelings. Moments of vulnerability—like defending Morinaga from his brother's prejudice—hint at unconscious attachment. His sister Kanako and Morinaga's friend Isogai observe and sometimes leverage his inconsistent behavior regarding their relationship.

His habits include heavy alcohol consumption during stress, smoking, and coffee drinking. Academically gifted with a Master's degree, he focuses on scientific research but procrastinates administrative tasks like reports, often delegating them aggressively.

His development shows incremental, often reluctant, shifts. While maintaining a homophobic stance externally, his actions with Morinaga—including negotiated physical intimacy and indirect concern—demonstrate evolving complexity. He remains resistant to labeling their relationship, fluctuating between viewing Morinaga as a necessary fixture in his life and recoiling from emotional acknowledgment.