Marie Karigari is an android created by Hiroshi Karigari, a college student and robotics genius. He modeled her after his romantic interest, a tennis club member named Marie. Designed to closely resemble Marie and pass as her twin, she possesses pink hair while the original Marie has blue hair, a visual distinction not explicitly addressed within the narrative. Her physical appearance is otherwise nearly identical to the human Marie.

She unexpectedly activates herself before Hiroshi completes her programming, causing immediate complications. Though initially programmed as Hiroshi's ideal partner, she develops her own distinct personality marked by deep affection for her creator and a childlike curiosity about the world. Her sentience enables independent emotional responses and decision-making, often diverging from Hiroshi's original intentions.

To conceal her artificial nature, Hiroshi introduces her as his sister, Marie Karigari. This cover story creates ongoing challenges as she interacts with others, particularly when encountering the original Marie and Hiroshi's confrontational acquaintance, Hibiki Kennou. Hibiki discovers Marie's true identity and blackmails Hiroshi into becoming her boyfriend under threat of exposure.

Her development involves exploring human experiences. Notably, she expresses a desire to dream, leading Hiroshi to modify her programming to enable simulated dreaming. During this process, she experiences ambiguous states between wakefulness and dreaming, creating uncertainty about her perception of reality. Her evolving consciousness raises questions about the nature of her existence and emotional capacities.

Throughout her interactions, she demonstrates increasing complexity in navigating relationships while maintaining the secrecy of her artificial identity. Her existence creates ongoing tension between Hiroshi's initial intentions and her autonomous development as a sentient being.