C.C. & KALLEN TOGETHER
C.C. and Kallen meet themselves in the 11th episode of the first season. Since C.C. was with Zero, Kallen begun to feel jealous of her. Their relationship wasn't developped that much in the first season, maybe because C.C. was always with Lelouch and rarely talked with the others.
In R2, we find out that their relation is very close. During the one-year period that they spent together, they knew more about themselves, their tastes and much more. They begun to talk about anything and, under Urabe's informations, they made Kuro no Kishidan's missions together (the most of this is in the Sound Episode 1 of R2).
C.C. is always the one who begins to make something and motivates Kallen (obviosly she does it in a sadistic way xD). We can see it clearly in the first episodes of the series, where Kallen wears a bunny costume and the main idea was C.C.'s. After she realized it, she thinks "why isn't her the one who had to wore this!?"! XD At the end C.C. talks quietly with her but she's very arrogant, and instead, Kallen is more aggressive, especially when C.C. tries to make her angry (she just need a sentence, it's very simple XD)!
They're comical sometimes, they make me laugh all the time.
They're both in love with Lelouch, yes! But I think that their love is quite different due to ther past and relation with him. I think that, even if it is not make on purpose, they fight for him but continue to trust each other. In fact, when Kallen finds out (thanks to Lelouch) that C.C. lost her memories, is very upset and sad.