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Tomboyish daughter of a wealthy family and an "appreciator of beauty."
Nagi is the track star at school who always gives her all at whatever she wants, whether it's exercising or swimming or whatever. Her boyish looks and small chest have led many people around her to mistake her for an actual boy. Apparently, this is why her first boyfriend quickly broke up with her, thinking that she was just a gay guy. As a result, she decided to go full-on tomboy.
Nagi first enters the scene in Nyan Koi! when she thinks that Junpei made Kaede cry, and challenges him to a duel over it. However, Junpei is unaware of Nagi's challenge, and Nagi ends up getting sick from waiting outside the whole day. After escorting Nagi back home, she challenges him to a duel again over Kaede's love, until an accident reveals that Nagi is actually a girl and that she is afraid of lightning. Her voice rapidly escalates in pitch and she locks herself inside a sarcophagus. After Junpei manages to calm her down with his words, Nagi suddenly grows a lot more fond of him and becomes a sort of rival love interest.
In the anime, Nagi continuously tries to help Junpei take the next step in his relationship with Kaede, warning that if he chickens out then Nagi will come after him. Even so, she still seems to harbor some feelings for him and possibly for the other girls as well (Nagi sometimes refers to Kanako as "D-cup" and grabbed her breast on a couple of occasions. Kanako didn't take it well). Also, aside from the fear of lightning it was revealed that she has an odd condition where she can get drunk on juice. It's not known if this is considered canon or just a one-off joke.