Masami is a young woman with red hair who becomes involved with Gawl, Koji, and Ryo shortly after they arrive from the future. She first encounters Gawl while working a part-time job at a fast food restaurant, mistaking him for a stalker who has been lurking in the area. This misunderstanding leads to a poor first impression that defines much of her early interactions with him. She lives with her mother, who rents out a room to the three time travelers, so Masami ends up sharing her home with them as they attend school together.

Her personality is curious and spirited, often described as strong-willed or tsundere in nature. She frequently clashes with Gawl, arguing with him over minor issues, but her behavior also hints at a growing romantic interest in him that she does not fully acknowledge. Her motivations are rooted in ordinary teenage concerns, but she is gradually drawn into the larger conflict surrounding the Generators and the mission of the three boys. She acts as a grounding presence, representing the normal world that the protagonists are trying to protect.

Masami's most significant relationship is with Gawl, defined by constant bickering that masks deeper feelings. She is also close to her pink-haired best friend Natsume, who often tries to mediate when Masami and Gawl argue. Over the course of the series, Masami develops from a skeptical and confrontational outsider into a more sympathetic and supportive ally, though she remains a civilian without any special abilities or combat role. She does not possess superhuman powers or any connection to the Generator experiments; her value to the story lies in her emotional connections and her function as a link between the extraordinary events and everyday life. Her growth involves learning to trust Gawl and the others, and her presence helps humanize the high-stakes narrative.