Eri Ninamori is the daughter of the mayor of Mabase and the president of Naota's sixth grade class. Like Naota she is obsessed with acting grown-up, but she often loses her composure when angry or excited. Ninamori hides her frustration with her father's sex scandal, and rigs an election for the school play so that she gets the lead role while Naota gets cast opposite her, hinting that she has a crush on him.
Eri Ninamori, 12, is Naota's classmate. In the series, she is referred to as 'Ninamori,' her family name. She is the daughter of the mayor of Mabase and is class president. Ninamori has N.O. powers as well, though they only manifest once when Naota hit his head on hers. In this instance, Naota's third robot, which expresses at first as "cat ears", transfers to her instantly when the Medical Mechanica factory's siren blows. In the manga, an octopus-like robot comes forth, before running away with Naota, and appears very little after that until it helps her save Naota from an angered Haruko.
Ninamori projects a proper, even slightly stuck-up personality, and conceals personal information to an almost obsessive degree; she does not even let her classmates know that she needs corrective lenses, instead wearing contacts while at school. Even with her class president status, however, Ninamori isn't above breaking the rules to get what she wants. She rigs the voting during the casting for the school play in order to get the lead role and have Naota cast opposite her, which hints at her crush on Naota. Other hints are her expressed dislike of Mamimi and Haruko, letting him see her wearing glasses and a final scene with Naota by a vending machine that mirrors Mamimi and Naota's early-series interactions. Ninamori is also highly intelligent, if a little jaded. Much like Naota, Ninamori attempts to act mature, and although her parents' problems trouble her deeply, she refuses to admit it until near the end of the series. During the final episode, during gym practice, she tells her classmates Masashi Masamune and Gaku Manabe that she had finally talked to her parents, letting them see her cry, and then performs an impressive high jump which her classmates and teacher applaud - a possible representation of how Ninamori has achieved real emotional maturity, something which Naota had yet to accomplish.
Ninamori is a bit tall for her age, tall enough that Naota's pajamas are a little short for her. She has dark, blue-green eyes and purple hair. For the first half of the series, she wears her hair long and straight. At the end of the third episode, it is cut to chin-length, and she styles it with accessories or in short pigtails for the rest of the series. In her last scene in the final episode, her hair has grown longer and she wears it in braids. Ninamori wears different clothes in all of her appearances and has many outfits throughout the series.
Ninamori's glasses are her most significant possession. Although she is only shown wearing them in episode 3, they represent her true self and the only person she allows to see her wearing them is Naota. At the end of the episode, when the school play is about to start, he sees her wearing glasses on stage, but she pokes her finger through the frames to show they have no lenses. These fake glasses are not the same as the ones she wore earlier in the episode, which have real lenses.
Ninamori seems to have a crush on Naota, first indicated by playful teasing and jealousy towards Mamimi. In the third episode, she goes so far as to rig the votes just to have him cast as her co-star in the class play, and even confides the secret to him when they are alone. Naota, meanwhile, treats her with a bit more respect than he does Mamimi or Haruko, sympathizing with her family issues and calling her "very grown up" at one point. They do not take any further steps toward romance, however, until the final episode.
Ninamori does not meet Haruko until the third episode, and her first impression of her is a strongly negative one. However, although it's clear Ninamori has some jealousy of her relationship with Naota, she doesn't do or say much about it later.
Ninamori does not interact with Mamimi at all, but from hearing her called "Naota's wife" and seeing them together, she is quick to judge her as a ditzy airhead. Her opinion of Mamimi seems to change slightly when she notices how Naota's closeness with Haruko is hurting her.
Probably the closest friends Ninamori has besides Naota. They call her 'Ninamo', she hangs out with them during and after school, and even gets a part-time job making deliveries with them. By the end of the final episode, they start showing signs of attraction to her, though she still seems more interested in Naota.