Hiro Iokawa was studying in America when his adoptive parents died, forcing his return to Japan for their funeral. While abroad, he formed a close bond with Shōko Nogami, who became his girlfriend. He intended to quickly return to America after the funeral.

The discovery of his adoptive younger sister, Karada, changed his plans. Despite sharing no blood relation, he chose to stay in Japan as her primary guardian, abandoning his overseas studies and relationship. Months later, he ended things with Shōko via a letter offering no real explanation, causing her confusion and hurt.

Financially supporting Karada required Hiro to take a job at a local pharmacy. He also managed their household, demonstrating a deep commitment to her care. Hiro is introverted, often shielding his eyes with his hair, and carries underlying guilt over abruptly leaving Shōko. His protectiveness toward Karada sometimes leads him to interfere in her social life, particularly concerning her classmate Tetsumasa Amino, who has romantic feelings for her.

The supernatural age reversal affecting Karada and Shōko deeply impacts Hiro. He initially fails to recognize Karada in her adult form and struggles to accept their transformation. His persistent search for the "missing" Karada shows his dedication, though he gradually adjusts to supporting both women through their changed circumstances.

The manga adaptation explores his relationships and emotional conflicts in greater depth than the anime, explicitly clarifying the non-biological nature of his bond with Karada and delving into unresolved tensions with Shōko. His character development focuses on reconciling past decisions with current responsibilities.