Samejima Mamimi (サメジマ・マミ実), 17, was Tasuku's girlfriend though she hits on Naota since his brother left Japan. She attends school infrequently and has pyromaniacal tendencies, smokes a lot, and spends most of her time under a bridge. It is likely that her parents are separated.
She has a rather peculiar tendency to name many things after her personal nickname for Naota's brother (and Naota himself) "Ta-kun." Japanese adolescents often add the suffix "-kun" to the names of their classmates and friends their age or younger. She calls Tasuku "Tasuku sempai" because he is older, but is likely using Naota as a substitute, and she refers to him as "Ta-kun" (which is short for Naota-kun, but also sounds suspiciously close to Tasuku. Naota is well-aware of the similarity between his nickname and his brother's name.)
It is originally set forth in the story that she has a fascination with fire after her rescue from the elementary school by Tasuku, though later it is revealed that she herself set many fires and is hinted that she was the one who started the fire in her elementary school. She is a high school student but is truant much, if not all, of the time.
As far as high schoolers go, she seems to be rather sexually frustrated, as she often pursues an uncomfortable level of intimacy with Naota (most likely because she sees him as his brother's replacement), which annoys Naota greatly. When he asks why she constantly clings to him, she responds "If I don't, I'll overflow."
At the end of the series, she leaves Mabase to become a professional photographer.
Most characters refer to Mamimi simply as Mamimi, but Naota's friends refer to her as "Naota's Wife" and Haruko calls her "Sameji", a shortening of her last name. Amarao refers to her as "that high school girl".