Face it, Kariya is the reason of all the rumors. He's the reason the others were food poisoned by Jelly beans. He's a total trickster.

He's also the reason why Kirino decided to dye her- i mean HIS hair from pink to black and cut it...

Kariya: Good morning senpai.

Kirino: Hey Kariya.

Kariya: Senpai... Are you a girl pretending to be a boy?

Kirino: Yea- wait, am i WAT?!

Kariya: A girl.

Kirino: NO! I'm a boy! Why does everyone ask me that?!

Kariya: Because of your hair.

And thus, this is why Kirino wanted to change... Unfortunately Shindou stopped her- i mean HIM, from doing so and he told her-HIM, to ignore Kariya.