X-2 wasnt part of the main series. It was an offshoot.

I know this thread is going to turn into the endless lectures about how VI has no real main character, but before it does I'll simply say that I feel Terra is the catalyst character. She's the reason for things starting to happen, really. If Terra wasnt there, would Tritoch have reacted? Would Locke and Edger ever have gone to seen Bannon at that time? You get what I mean.

So yeah. Catalyst character.