Tatsuya Tsugawa is a college student and boxer whose aggressive nature manifests in his relationships. He initially pursues casual dating, meeting Eiko during a group outing and developing an emotional attraction. He expresses his feelings through unconventional and provocative actions. Their relationship intensifies with passionate encounters, including intimate moments on a sailboat, which awaken deeper emotions within Eiko and lead to her devotion. Tatsuya exploits this devotion, displaying cruelty and manipulation.

In a significant betrayal, Tatsuya brokers a transaction where his brother, Michihisa, "takes over" Eiko, selling her for five thousand yen. Discovering this arrangement, Eiko repeatedly repays the money to Michihisa whenever Tatsuya renews it. Later, Eiko informs Tatsuya of her pregnancy with his child. He reacts ambivalently at first but later demands an abortion after seeing a newspaper photograph of a boxer holding a baby. Due to the advanced pregnancy, Eiko undergoes a late-term Caesarean operation and dies from peritonitis four days later. At her funeral, Tatsuya interprets her smile in a photograph as a challenge and angrily throws a container of incense at it. He retreats to his college gymnasium to box, recalling her question about why he couldn't love her straightforwardly.

In the 2002 miniseries adaptation, Tatsuya is portrayed as a tormented college student, once considerate but now harboring a significant grudge. He befriends the wealthiest student at his school, a trusting individual, while secretly intending to destroy him. His actions include defacing the friend's new car and pursuing his fiancée, though his specific motivations for these actions remain unclear.

The character also appears in other adaptations, such as a 1986 animated film and a 2011 eroge, but details of these portrayals remain unspecified.