Onii-san is a recurring salaryman character who first encounters high school student Ai during their shared commute on crowded Monday morning trains. He routinely shields her from the jostling crowds, establishing a weekly ritual where Ai gives him small charms, most often buttons popped off her shirts due to her large bust size. This routine forms the foundation of their relationship, which gradually evolves from casual acquaintance into a deeper connection. He serves as a dependable and protective figure in Ai's life, offering emotional support and helping her build confidence regarding body image struggles stemming from unwanted attention. Their bond deepens into a romantic relationship over time, conveyed more through implication than explicit narrative.

His familial connection is revealed through Volley-bu-chan, identified as his niece and Ai's best friend, who occasionally teases Ai about her figure while also being protective. He maintains professional relationships, notably with his boss (Chief Manager or Kachō), and interacts with other characters within the shared setting, such as residing near the Teacher and Maegami-chan after they move. His background lacks extensive elaboration, with no specific details provided about family history, upbringing, or life outside his interactions with Ai and his job.

Across the series' official media, including the manga spanning multiple volumes and anime seasons and OVAs, his role remains consistent as Ai's primary love interest and a stabilizing presence. He appears in numerous manga chapters across volumes 1 to 11 and in episodes spanning both anime seasons and OVAs. Defined by kindness and a protective nature towards Ai, significant internal development or major life changes are not depicted. His character primarily facilitates Ai's journey towards self-acceptance and anchors the series' recurring theme of age-gap romances, a pattern also observed with other female characters and their older partners.