Saki Akutsu is Mamoru Suzuhara's high school classmate and a member of the group joining Aya Chiyono's seven-day stay at the abandoned Satomi Coal Factory. She possesses a naturally cheerful and spontaneous personality, generally avoiding deep thought except when it comes to her hairstyle. Saki's main reason for participating is her desire to turn Aya's birthday into a memorable event; specifically for this purpose, she adopts a new hairstyle believing it suits Aya's rebellious phase.

She extends an invitation to classmate Hiroto Honjou to join them, despite his lack of closeness to Aya, deceiving him into attending by claiming she has something important to tell him. Saki and Hiroto have known each other since kindergarten, and she holds him in high regard, considering him smarter than Mamoru. Her description of Hiroto as a studious person with few interests is meant as playful teasing, not malice.

Perceptively, Saki identifies Mamoru's romantic feelings for Aya and views their hideout as his attempt to pursue her. She confronts Mamoru directly about this and issues a protective warning, stressing Aya's innocence and vowing never to forgive him if Aya gets hurt.

During the group's stay, as members share personal secrets, Saki discloses comparatively minor vulnerabilities: jealousy over Aya's impending move to Tokyo and anxiety about gaining weight during their week confined together.