Nanako Momoi is a high school girl who appears small and cute, but her outward appearance is deceiving. Once she speaks or acts, her true nature emerges: she is highly impulsive, violent, and brash, with a personality that many describe as tomboyish. These traits cause most of her classmates to be repelled by her and even fear her. She is often called a true tomboy and is known for her rude and obnoxious behavior.

Nanako’s background involves her grandfather, a mad scientist figure who conducts bizarre experiments. One day, Akira Uehara, a shy and effeminate classmate who has a secret crush on her, visits her home to deliver homework. An accident involving her grandfather’s invention causes Nanako and Akira to switch bodies. From that point, Nanako finds herself in Akira’s male body.

Her motivation shifts dramatically after the body swap. Unlike Akira, who desperately wants to return to his original body, Nanako revels in her new situation. She enjoys being a boy and actively seeks to remain in Akira’s body for as long as possible. She uses her new form to engage in activities that suit her bold and confident nature: she flirts with older girls, goes to hotels with them, and even masturbates, much to Akira’s horror. She threatens Akira not to do anything inappropriate with her original body, yet she shows no restraint in abusing his.

In terms of role in the story, Nanako is the central female character whose body switch drives the romantic comedy plot. She is the primary source of conflict and humor, as her aggressive and carefree attitude contrasts sharply with Akira’s meek and sensitive personality.

Her key relationships include Akira Uehara, with whom she shares a tense but co-dependent bond after the swap. She also develops a romantic relationship with her best friend, Makoto Shiina. As a boy in Akira’s body, she becomes attracted to Shiina, and the two eventually start dating. This relationship is mutual and becomes a significant part of her character development. She also interacts with Shinnosuke Senbongi, Akira’s friend, who becomes romantically interested in Akira in Nanako’s body.

Over the course of the series, Nanako undergoes development. Initially, she is purely selfish and enjoys the freedom of being male without concern for Akira’s feelings. However, as the story progresses, she grows to value her relationship with Shiina and becomes more invested in her new life. Later, when the pair temporarily revert to their original bodies due to another experiment, Nanako is forced to readjust, but ultimately the story explores the idea that the swap may have been a better fit for both their personalities. In the extra chapters, years later, Shinbongi successfully rebuilds the swap device, allowing the couples to reunite properly, and Nanako ends up marrying Shiina.

Notable abilities: Nanako has no supernatural powers, but she possesses exceptional athletic ability and confidence, which make her popular when she is in Akira’s body. Her strong will and lack of inhibition allow her to navigate social situations that Akira would find impossible. She also demonstrates a surprising capacity for emotional growth and commitment, as seen in her lasting relationship with Shiina.