Kosuke initially appears as Shizuka's childhood friend during the week before her family's move, forming a significant early connection that leaves a lasting impression on her.

Twelve years later, Shizuka returns to Tokyo at age seventeen specifically to reunite with him. Now exhibiting a markedly lecherous personality, Kosuke engages in frequent sexual encounters with multiple women, often within exaggerated and comical scenarios that cement his reputation as a womanizer.

Despite this overt promiscuity, Kosuke sporadically displays moments of decency or kindness. These fleeting glimpses of a better nature lead Shizuka to maintain hope that his underlying character can be reformed. She believes these flashes indicate his potential for a genuine commitment, motivating her persistence in their renewed connection.

His narrative development centers on the tension between his established behavior and Shizuka's efforts to influence him, exploring whether his sporadic acts of decency can solidify into consistent change, particularly in response to her presence and expectations.