At first, Ryouko appears to be a compassionate and hard-working high school girl. One of Kyon's friends, Taniguchi, places her in the top three first-year girls in the school with a grade of AA+: indicative of her pleasant personality, beauty, and athleticism. Her enormous popularity made her the unanimous choice for class president of Kyon's homeroom class. She also showed particular concern about how Haruhi had distanced herself from the rest of the class.
However, it turns out that she is Yuki's backup, and as such, is also another "humanoid interface." Much like Yuki, she finds it difficult to empathize with humans, but unlike Yuki, possesses exceptional social skills. Initially she cheerfully supports Kyon's reaching out to Haruhi, often encouraging him to "open her up to the class," making Haruhi easier to observe. However, she later decides to try and kill Kyon with a combat knife in the hopes of catalyzing an observable reaction from Haruhi. Her efforts are stopped, after Yuki defeated and removed her from the physical world. It's interesting to note that her cheerful personality doesn't falter, even at the moment of her own demise. Afterwards, Yuki fabricated a cover story that she moved to Canada.
Ryouko makes a reappearance in the Disappearance arc, on an alternate timeline in which Haruhi never attends North High or forms an SOS Brigade. In this timeline she is just a normal human and is very kind to the shy Nagato; however, this Ryoko's timeline is obssesed with her; when she seemingly sees Kyon trying to shoot Nagato with a gun, she stabs him with a knife, believing that this is what Yuki wanted, it was briefly shown that Ryoko may show feelings for Nagato and would do anything to protect her.
In the Astonishment arc, Ryouko takes her rightful place as Nagato's backup since Nagato became ill. While she still wishes to kill Kyon (and even goes to far as to tell him this with a smile on her face), she ironically saves him twice from Kuyou. Kyon made the decision that she should not enroll at North High again (as he doesn't want to provoke Haruhi) which she agrees to. Ryouko is currently unable to attack Kyon because Emiri was given special counter-measures against Ryouko's attacks. Because of this, Ryouko is actually a bit of an ally to Kyon at this point.
Fighting is bad... but sometimes it can be so bad that it's good, right? Prepare yourself for some serious ass-kicking. We're counting down 20 of the greatest, coolest, and most epic anime fight scenes in anime history.