Aiko Yoshinaga is a main character and photographer. She first meets protagonist Ken Ferrari in Tokyo when he assists her while she is stranded with car trouble. This encounter sparks a connection between them. Their relationship deepens into a close romantic and physical involvement. Together, they experience events like a getaway from pursuing bikers after Ken drives provocatively past them. The narrative implies at least two instances of off-screen physical intimacy between Aiko and Ken.

Aiko holds significant prominence during the story's first half, serving as a major figure in Ken's life, with Ken showing clear attraction to her physical appearance. However, her narrative presence diminishes substantially following the introduction of Eri Hayami. Once Eri becomes involved with Ken, Aiko receives significantly reduced focus within the plot's progression.