[Undertaker]

While the main series did not really state that, the drama CD that was released did imply her potential yandere side.

Spoiler for drama cd:

Basically Kirino got recently got into "Love Touch" [parody of Love Plus DS game] and got caught playing it in school by Ayase and subsequently openly play the game in front of her. Feeling ignored, Ayase asks Kyosuke for advice. Long story short, Kyosuke gor Ayase to act and talk like the game character, and along with Minami discovered that Ayase somehow opens up the ultra secret yandere scenario....