Aiko Mutō is a high school student at Toumei High School with a slim, athletic build, standing 168 cm tall and weighing 48 kg. She has auburn-brown hair reaching mid-back and dark brown eyes. Her ears feature a distinctive arrangement: two red studs on the right and three silver hoops on the left. This current physique resulted from significant past weight loss.

Before high school, Aiko had a heavier build, weighing approximately 40 pounds more, and suffered severe acne. She wore curly hair held back by a black headband and heavily used beauty products to conceal her skin, inadvertently worsening her complexion. Driven by a desire to satisfy boyfriends and gain social acceptance, she embarked on drastic dieting, losing 17 kilograms over two months. This rapid weight loss left lasting stretch marks on her stomach, fueling persistent body image insecurities.

Aiko projects a bold, straightforward personality with strong protective instincts toward those close to her. She often presents an initially brash or mean-spirited demeanor, gradually revealing a caring and supportive nature after trust develops. Her tendency to erect emotional "walls" frequently limits deeper connections until she feels secure.

Her romantic history involves multiple superficial relationships, typically ending with partners cheating. During a vulnerable period following one betrayal, Yamato Kurosawa comforted her, praising her natural appearance without cosmetics—a first for Aiko. This prompted her to request physical intimacy, which Yamato agreed to in an attempt to bolster her self-esteem. Consequently, she considered Yamato her "first love," nurturing enduring but unreciprocated feelings.

Entering high school, Aiko harbored intense jealousy towards Mei Tachibana due to Yamato's romantic involvement with her. She initially perceived Mei as undeserving of his affection and actively doubted her sincerity. Aiko's perspective shifted after witnessing Mei defend her against a boy's mocking remarks about her weight-loss history. This act prompted Aiko to reevaluate Mei, leading to a gradual friendship. Aiko later alerted Yamato when his secret interactions with Megumi Kitagawa caused Mei emotional distress, demonstrating protective loyalty.

Aiko eventually moved beyond her fixation on Yamato and entered a relationship with Masashi Tachikawa. He accepts her unconditionally, including her stretch marks and past insecurities, becoming the sole person with whom she feels comfortable being physically vulnerable. Despite this security, residual self-consciousness occasionally surfaces.

Her given name, Aiko (愛子), translates to "love child," while her surname, Mutō (武藤), signifies "warrior wisteria," historically connecting to the Fujiwara clan.