Ryoko Asakura

Ryoko Asakura was a major antagonist in The Melancholy of Haruhi Suzumiya and a minor villain in the third arc of Pretty Soldier Sailor Stellar.

The Melancholy of Haruhi Suzumiya
Ryoko first appears as 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 Suzumiya 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. Afterward, Yuki fabricated a cover story that she moved to Canada. Ironically, the death/disappearance that provoked Haruhi's reaction in the end was Ryoko's own.

The Disappearance of Haruhi Suzumiya
Ryoko does not appear again until The Disappearance of Haruhi Suzumiya, in the alternate reality, in which she appears as a normal human. She sits behind Kyon, where Haruhi once sat, and cares for Yuki as "a friendship fostered by living in the same apartment block". Because Yuki has difficulty keeping herself active, Ryoko is forced to give her food every day and help her with minor things. However, Ryoko is still homicidal; when she seemingly sees Kyon trying to shoot Yuki with a gun (he is actually trying to revert her to her normal Data Humanoid Interface self), she comes after him with a knife the same way she tried to kill Kyon in The Melancholy of Haruhi Suzumiya, believing that this is what Yuki wanted. After a Yuki from the near future comes and saves Kyon, she reverts the world back to normal, making Ryoko disappear.

Trivia

 * Ryoko had a ponytail in her initial character design, but was later removed due to Kyon's fetish.