Risa Momioka is Haruna and Mio's best friend and Rito and Lala's classmate. She is very fun-loving, mischievous and perverted. She enjoys talking about naughty things, teasing Rito, and her favorite thing to do is groping the breasts of other girls and whisper naughty things into their ears to tease them.
She is also part of her school's tennis club like Haruna, but she mostly skips it to do other things like karaoke.
Risa is an attractive girl with dirty blond colored hair and dark brown eyes. She has a curvaceous figure (though not at the level of Lala or Yui), and her 3-sizes are B83-W60-H85. She sometimes has trouble keeping boys at bay. Of all her classmates she is probably the most willing to use her figure to attract attention, save for comedic scenes.
It's also notable that when in school, she does not wear a tie, which is included and supposedly required with the uniforms of female students.
Risa likes wearing her school uniform even on days off, seeing it as a brand itself.
Risa is quite indecent, as she always goes around groping other girls' breasts and suggesting how something would lead into an erotic situation. She is also enjoys teasing other people through about sexual things. She is quite nosy and likes to joke about other peoples' personal business like their love lives, like when she asked Yui if there is someone she has in mind or giving Lala bad advice about her relationship with Rito. She and Mio tend to enjoy anything "ecchi" that happens around them, like Mea's Trans-ability to increase her bust size. Risa is in fact so amazingly perverted that even Momo is impressed, intimidated, and vexed by her technique when Risa gropes her in the shower.
It's entirely possible that Risa may be bisexual, as evidenced by her admiration and magnetic attraction to the female body, as well as her deeply-implied feelings for Rito.
Risa is a very mischievous and perverted girl. And so, she likes to be around shy and unwilling people whom she can tease and play with easily.
It is not known what Rito and Risa feel for each other, but it could be assumed that they are generally friends, or at least acquainted through Haruna. While various other girls consider Rito to be a pervert, Risa knows that he is an innocent boy to a fault and likes to tease him about sexual subjects.
Rito wishes that the naive Lala would stop taking lewd ideas from Risa and Mio. And Risa doesn't take Rito's rejection of sex and temptation seriously, and likes to play with his shyness around girls and occasionally gropes the breasts of other girls in front of him, knowing that she's making him feel uncomfortable.
Very early in the story, she didn't think much of him as anything special, but It has been shown that Risa might have feelings (or at least lusts) for Rito, but it hasn't been detailed if she was serious or joking. In chapter 147 and episode 10 of Motto To Love-Ru, she takes a much closer and bolder approach in teasing him, but it is still not clear what her true feelings for him are. She has gone on record that saying he can be helpful, innocent and even cute when he overreacts to things. In a To Love-Ru Darkness side story, when Momo asked her what type of guy she likes, she answers it was someone like Rito.
She is good friends with Lala and likes groping her breasts. Risa often gives wrong or lewd ideas to her questions about how to get Rito to like her more, something he really dislikes about the two.
Like with any other girl, Risa likes to tease and play with Momo. She is practically shown to be even more indecent than Momo herself, being able to make her react when she was molesting her. She has stated that she likes that reaction of hers, comparing it to Rito's.
Momo herself is also very perverted and enjoys sexual pleasure, though only by Rito. Nevertheless, it's surprising that she, of all people, would be so overwhelmed when she was 'hit on' by Risa.
Risa's friend and partner in crime, she also enjoys groping girls' breasts and whispering in their ears.
Risa's friend, whom she occasionally gropes along with her friend Mio, sometimes saying they've "grown", mostly in front of Rito. Haruna would get mad at her for when she talks about erotic things, especially when she involves Rito in it.
Risa is one of the people that Yui has to tolerate the most because of her indecent behavior, like groping her's or other girls' breasts, claiming her and also Mio shameless. However Risa also calls Yui shameless for having an attractive body. Risa would sometimes tease her about these sorts of things which further angers her.