Rei Ikegami is the original owner of Spank before the dog bonds with Aiko Morimura. Reclaiming Spank early on, Rei sees the dog escape back to Aiko, leading him to relent and allow Spank to stay. Rei possesses a deep connection to the sea, owning a yacht central to his identity. This parallels the disappearance of Aiko's father at sea years earlier. His own father, a doctor running a Tokyo hospital, represents family expectations contrasting with Rei's personal passions.

He develops a close relationship with Aiko. When his yacht encounters a storm and he goes missing, Aiko searches desperately. Found unconscious and hospitalized, Rei is transferred to his father's hospital. He confides in Aiko his father's plan to destroy his yacht, entrusting her to protect it. After the yacht is burned, Rei initially blames Aiko before learning the truth from his father. Their bond deepens when Rei recovers from a serious illness, aided crucially by Aiko's support. Rei also accidentally caused the death of Aiko's previous dog, Papi, in a traffic accident, creating initial resentment from Aiko that evolves into affection.

Rei passionately pursues music, particularly classical piano. This ambition drives him to study abroad, first in Vienna in anime continuity. His departure creates emotional tension, especially with Aiko, who encourages his dreams despite her feelings. Before leaving, Rei helps resolve Spank's future by advocating for the dog's autonomy, ensuring Spank stays with Aiko when she moves to Tokyo. Manga continuity offers an alternate path where Rei enrolls in a naval academy university.

He reappears sporadically, such as during a group trip to Tateshina plateau where interpersonal tensions surface. His father attempts reconciliation during this trip, but Rei rejects him until Aiko intervenes. His character arc concludes with him supporting Aiko's new beginnings while pursuing his own ambitions abroad, leaving a lasting impact on her personal growth.