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Friend of both Mio and Haruna.
Risa is one a duo that (including Mio) pretty much runs around the school groping and taking measurments of the other girls. A rumor mongerer and trouble causer, Risa teases and goads the girls around her with various lewd situations. One of Risa's favorite targets happens to be Haruna and when the class has gone to a summer resort (manga only) it is Risa that comes closest to discovering that Rito had become trapped beneath Haruna's blanket after Saruyama attempted to spy on the other girls.
Risa slips in and out of the manga and anime but never really has a significant role in the show. She does happen to have a rather late episode in the manga where, while being harrassed by a bum, grabs Rito and acts like she was his girlfriend to get rid of the bum. Risa then drags Rito to a cos-play resteraunt where Mio works and makes Rito pay for lunch. Afterwards Risa rather pushes Rito into walking her home. Upon arrival she forces him to come inside (anyone starting to notice a pattern here?). In her room she innocently asks how far he had gotten with Lala (although it is VERY apparent what she is actually asking). When Rito flips out and denies anything of the sort Risa tackles him onto her bed and starts to whisper sweet nothings in his ear. Rito is of the impression that Risa was about to have sex with him but before Rito passes out Risa starts to laugh hysterically and remarks that his reaction was very cute. Rito then makes good on his opportunity to sprint out the door. On the way he gets an e-mail on his phone that states very clearly, if anything ever goes wrong with Lala Risa would be quite happy to "teach" Rito anything and everything. It is never clarified if Risa is actually interested or was just teasing Rito.