Mamimi was Tasuku Nandaba's girlfriend until he left Japan. She was then seen hanging around Tasuku younger brother Naota who is the protagonist of FLCL.
Mamimi Samejima, 17, was Tasuku Nandaba's girlfriend, though she dotes on his little brother Naota Nandaba since Tasuku left Japan to play baseball in the United States. She is a high school student, although she seldom attends. She has pyromaniacal tendencies; she also smokes a lot and spends most of her time under a bridge. She is often seen with a camera, and smokes cigarettes with "Never Knows Best" written onto the paper.
She has the tendency to name people and things Ta-kun (sometimes spelled as Takkun), after her nickname for Naota's older brother Tasuku. The "-kun" suffix is an informal and intimate honorific primarily used when addressing males of equal or junior status. Mamimi is likely using Naota as a substitute for Tasuku and therefore refers to him as "Ta-kun" which can be short for both Tasuku and Naota; Naota is well-aware of the similarity between his nickname and his brother's. Mamimi also calls Tasuku Tasuku-sempai since he is an older classmate.
The story originally sets forth that her fascination with fire stems from Tasuku having rescued her from the elementary school fire, although it is later hinted that she was the one who started that fire. Mamimi holds a weak grip on the difference between fantasy and reality; for example, she believes Canti to be the manifestation of Cantido, the video game god to whom she burns things as sacrifices. She also emanates an aura of being rather dull that causes much of the cast (Naota's classmates especially) to look upon her with disdain. In fact, Mamimi can be exceedingly destructive - although only when nobody is watching. She spends much of FLCLimax exacting retribution against those who have wronged her - only to have her plan backfire.
Mamimi can be seen to lack control over her life or willpower, hence the near-constant smoking and child-like reliance on others, whether it'd be Naota or his older brother Tasuku. This could also explain why she loves taking care of things that are weak-willed, which she leaves (or which escape, like Ta-kun, the black kitten) when they start to show independence. This leads to her eventual disinterest in Naota and ultimately proves a disaster for her when her last adoptee, the terminal core from a robot, goes haywire and overpowers first her and then the military.
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."
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".