Mamimi Samejima is a seventeen-year-old high school truant who was the girlfriend of Tasuku Nandaba before he left for America. She is a lonely and depressed chain-smoker who rolls her own cigarettes with the phrase Never Knows Best printed on the paper. She speaks with a distinctive verbal tic, adding ssu to the end of many sentences. Mamimi has pyromaniacal tendencies, which are linked to a traumatic childhood incident: when she was twelve, a fire destroyed the town elementary school, and Tasuku rescued her from it, though it is later hinted that she herself may have started that fire. After Tasuku departs, she gravitates toward his younger brother, Naota, addressing him as Ta-kun, the same nickname she used for Tasuku. This habit extends to other living things; she adopts a series of pets and names them all Ta-kun as well.

Mamimis personality is complex, marked by emotional vulnerability and reckless behavior. She appears air-headed and vapid, which leads many of Naotas classmates to disdain and bully her, but her detachment conceals deeper emotional struggles. She has a tenuous grip on the boundary between fantasy and reality, as seen when she mistakes the robot Canti for the video-game deity Cantido and offers sacrifices to it. Though she often seems passive and childlike, she can be destructively aggressive when she believes no one is watching. During the events of FLCLimax, she attempts to exact revenge on those who have wronged her, only for her plan to backfire.

Her motivations are rooted in a deep sense of abandonment and a longing for emotional stability. She lacks control over her own life and willpower, which she compensates for by seeking out weak-willed beings that she can care for and dominate. However, when those beings begin to show independence, she loses interest and abandons them. This pattern is reflected in her relationship with Naota: she initially clings to him as a replacement for Tasuku, pursuing an uncomfortable level of intimacy that annoys the boy. When asked why she does this, she replies, If I dont, Ill overflow. As Naota grows more self-assured, she gradually loses interest in him.

Her key relationships define her arc. Her bond with Naota is complicated, shifting between affection and irritation, and is driven largely by her emotional dependency. Tasuku represents both safety and an unattainable ideal, and her inability to move past him stunts her growth. Haruko Haruhara dismissively calls her Sameji, while Naotas friends refer to her as Naotas Wife. Throughout the series, her story reflects themes of alienation, trauma, adolescent confusion, and the struggle for identity in the face of change. She possesses no supernatural abilities, but her blurred perception of reality serves as a metaphor for her internal turmoil.