The head of the school disciplinary committee, as such she is disgusted by the situations that Rito inadvertantly gets himself stuck in.
Yui Kotegawa is the head of the school disiplinary commitee, as such she is disgusted by the situations that Rito inadvertantly gets himself stuck in. Regardless of whether or not it was his fault (which it really never is).
In the anime she gets so frustrated by Rito that she goads the principal into making it so that Rito is not allowed to converse with girls for a week. After that Yui watches the fruits of her labors growing conflicted after watching Rito perform various acts of kindness for no reason at all. Typically, Yui starts to feel bad about what she did and apologizes to Rito.
At a resteraunt across the town however, Lala was planning a "cheer up Rito" party and uses one of her gadgets to bring Rito there. A super magnet that draws Rito against his will with no regards to whatever might be in his way. In a typical ecchi fashion, Yui is in the way and Rito pretty much flies across town with his face buried in Yui's chest and his hands cupping her rear end. Needless to say Rito gets slapped really, really hard.
A common event between both anime and manga is when Yui confiscates one of Lala's gadgets, made at the request of Risa and Mio, that was designed to make the uniforms shorter. In essence it turns Yui's modest uniform into somthing one might see on a playboy mansion spreadsheet. In the manga however, one added bit was that Rito is on the negative end of another one of Lala's inventions, one designed to get the boys "closer" to the girls. In effect this device acts like Lala's super magnet and throws him on Yui in her "shortened" uniform. Rito gets smacked.
An event exclusive to the manga, Yui is home sick. Rito is asked by one of the other school government kids to bring a memo to Yui. Rito arrives and is greeted by Yui's older brother who, (fun fact) is attempting to date Haruna's older sister.
Rito enters Yui's room... and is greeted by one of Yui's guilty pleasures, Yui loves cats. Her room is plastered with cute cat pictures and other such objects. Even the chocolate that she makes for Rito on Valentines day is in the shape of a cat (but that occurs later). Yui attempts to be her usual self but her illness negates those efforts fairly quickly when she falls out of bed and Rito catches her before she hits the floor. Predictably, Rito is groping Yui's breasts. This time however Yui admits that it was her fault and doesn't get angry at Rito.
Yui is one of the girls who frequently "bares it all" at the discretion of the writer. This does inittially cause friction between her and Rito but she slowly starts to not mind "as much". Even getting to the point where she admits to herself that she loves Rito.
One last interesting tidbit... Yui can't swim. So, whenever water is involved she always has a float ring with cats on it (dun dun dah).