Koroshiya-san stands as the central figure in his series, acclaimed as Japan's premier hitman. He lives by the motto "If I have an assignment, I will kill anything," applying it not only to conventional targets but also abstract or mundane tasks like eliminating summer vacation homework, household chores, or other non-human objectives. This expansive interpretation sparks comedic scenarios where his lethal professionalism clashes with absurd requests.

His personality merges stoic efficiency with unexpected quirks. While maintaining a serious, focused demeanor during assignments, he displays social awkwardness in routine situations—blushing during haircuts or becoming flustered in mundane interactions. This contrast between his deadly occupation and off-duty behavior amplifies the series' comedy. He occasionally performs unconventional actions, such as cleaning a target's residence post-assignment or sharing a dirty magazine with his apprentice, highlighting his eccentricities.

Interpersonal dynamics reveal further complexity. His mentorship of an apprentice covers both assassination techniques and everyday absurdities, though the apprentice’s role serves comedy over development. Antagonistic interactions with a persistent detective often devolve into unintentional homoerotic tension or mutual awkwardness, shifting abruptly from confrontation to uncomfortable personal moments. During missions, he sometimes breaks protocol for bizarre or nonsensical exchanges with targets before fulfilling contracts.

Concrete details about his background, origins, or personal history remain absent. The character undergoes no significant development across the series, preserving a consistent persona centered on situational humor rather than narrative growth. His role focuses on delivering deadpan reactions to increasingly ridiculous scenarios without evolving beyond his established traits.