Originally thinking he was just another "Shameless" boy, she realized how wrong she was. Her feelings for him began to change to move towards romance when Rito saved her from a group of delinquents. Throughout the series, Yui's love interest in Rito developed. Yui made chocolate for him on Valentine's Day, even though she claimed it was just "Giri (Obligatory) Chocolate," she really made chocolate only for him. In the OVA, at a group picnic, Yui told Rito "I won't let you do shameless things with anyone else but me!!" while she was under the influence of pollen that caused everyone to freely speak their minds.
In To Love Ru: Darkness, while Momo is trying to further her Harem Plan, she manipulated Yui and Rito into being alone together at Rito's house during what appeared to be a rain. During a black out, Rito falls onto Yui. While Rito is on top of her on his bed, Yui puts her arms around him and explains how she has learned to accept that it is normal for boys to be shameless, and that they are not all bad. She then offers herself to him if he agrees to do shameless things only to her (chapter 4). Rito breaks away and explains to her that although he is interested, he does not do shameless things intentionally. Yui loves Rito even more as she sees his innocent nature and how he is different from what she perceived all boys to be. She surprised herself with what she was doing and how far she went, and is grateful to Rito for not seducing her.
After a little longer she comes to realize that she loves him. Although she still has got to put up with a lot of his clumsiness, she knows most of all that he is not a boy who would intentionally disrespect girls or take advantage of them. When Momioka playfully and dishonestly explains her sexual experience with Rito, Yui knows immediately that it was not true, remembering Rito could have done it with her when they were in his bedroom but did not, and thinks to herself that he is indeed different from other boys, and that he is special. Yui would prefer for Rito to be more responsible for his actions. She even mentioned that because he is careless, he brings shameless incidents upon himself.
While Rito does not have any romantic feelings for Yui, he does, however, find her attractive as shown when he gets embarrassed when being alone with her in her room. Her growing love for Rito surprises even herself. However, being inexperienced with boys and being a typical tsundere, she has difficulty opening up to Rito and often hides her shyness with aggression and anger.
While Yui is generally very uptight, and in some senses Lala's complete opposite, she still is her friend. Lala's outgoingness with Rito often leads to Yui considering it "Shameless," however it has become a norm with her. Yui shows a slight jealousy towards how close she is with Rito, but strongly denies it herself. While Lala likes to consider everyone to be her friends, Yui and Lala don't generally bond with each other personally.
However, despite all their differences both sports nearly identical curvaceous figures and together are considered to have most attractive bodies on school by others girls. Whenever a comparison of girls' figures is brought up Yui and Lala are often used as sort of ideal that other girls want to achieve. Ironically, even though Yui and Lala have the best bodies, they are the only two so far who pay no real attention to their appearance.
Yuu is Yui's older brother and a lady's man, which is something that Yui dislikes about him. Yuu is aware of Yui's love for Rito, which she denies. He is eager to see his sister grow to become more mature and acknowledge her love for Rito.
There is little evidence that Yui is explicitly friends with Risa and Mio. Because they are always groping other girls, talking about sexual subjects, and just being "shameless", Yui would naturally reprimand them to stop being so shameless, which is clearly not well heard by them. It does not help that Yui has a surprisingly attractive figure, which they pointed out as having a pretty "shameless" body herself. They would occasionally grope her body as well, like they would to other girls.
Because of how Risa appears more frequently, she practically annoys Yui the most, because of how she always comments on how mischievous she can be and how she would say something about a situation would lead to a perverted one, mostly involving Rito. She even lied to her about having done something "shameless" with Rito.
Celine has shown a possibly strong liking to Yui, as pointed out by Rito when she hugged her. Yui was once mistaken for being Celine's mother and in a way as Rito's wife to which she greatly denies, because of this Yui would sometimes have day dreams about the three of them as a family (at the same time being pregnant with Rito's child). On some occasions Yui gives Celine cat based books to read and sometimes she is seen reading to her at school.