Shunka Koito, granddaughter of the school founder attended by protagonist Chizuko Mikamo, initially greets Chiko with hostility fueled by envy of her intelligence, wielding her family status to dominate peers. Her demeanor transforms upon discovering the thrill of Chiko's adventures involving the phantom thief Nijū Mensō. Driven by the knowledge that her life will irrevocably shift upon her sixteenth birthday due to an arranged marriage, Shunka actively seeks amusement and excitement, determined to maximize enjoyment while she can.

She habitually escalates dangerous situations for her own entertainment, often ignoring potential fallout. This includes deliberately worsening conflicts during perilous events, such as an incident aboard a vulnerable submarine. A childhood tragedy where her twin sister drowned underpins Shunka's actions; she carries deep, unresolved guilt, believing she failed to prevent the death. This trauma feeds her thrill-seeking and aversion to confronting emotional pain.

Her character arc subverts initial expectations about the arranged marriage, revealing genuine affection for her fiancé later on. Through her evolving friendship with Chiko, Shunka gradually processes her guilt and matures emotionally, moving beyond a sole focus on fleeting thrills. She serves as a companion and support for Chiko during investigations into Nijū Mensō's disappearance, though her preference for dramatic outcomes sometimes complicates scenarios. Shunka evolves from leveraging social influence to mask insecurity towards finding purpose through meaningful relationships.