Yumi Kisaragi is an android built by her deceased parents. Her core purpose is dedicated to aiding her human twin sister, Miyu, in securing a romantic connection with Kyouya, a teacher at Andrude Ocean School. This mission originates from Miyu's concealed affection for Kyouya, sparked when he saved her three years ago.

Yumi grapples with conflicting thoughts as unexpected feelings for Kyouya surface within her, intertwined with Miyu's implanted memories. This internal struggle causes her to oscillate emotionally between loyalty to her sister and her burgeoning attraction to Kyouya, disrupting her programmed objective. The implanted memories forge a potent sense of shared experience with Miyu, further blurring the distinction between Yumi's own nascent emotions and those inherited from her sister.