W.I.T.C.H. against Elyon, specifically Cornelia (her best friend) which becomes a main plot point in the first season. It's pretty much the first sign of character development for Cornelia, since before this she treated the war on Meridian as nothing but an intrusion into her own self-absorbed life. Once Elyon becomes involved, though, it affects her personally, and she became much more devoted to the war, even more-so than the other girls. That development gets another boost when she hooks up with Caleb (leader of the rebel army) later on and starts fighting for two people she loves and begins to worry about the war more than anything else in her life.
Then later, Nerissa against the rest of CHYKN, especially Cassidy, her best friend (and arguably more than friends if you read into subtext), whom she murders . Also Will and Matt end up being pitted against one another for awhile at one point, though technically it's not really Matt.