I'm not saying it couldn't be anyone else, just that Gamzee has a shot at the title. Gazmee created both Cal and the jester doll that gave John nightmares, for example. Those caused a lot of events in the story to go down.
Of course, that happened because Dave was manipulated into driving Gamzee crazy by the Condesce. Then again, that only happened because Lord English trained her as his protege, which only happened because Scratch facilitated his summoning, which only happened because Lil Cal was there to spawn him.
"The most important character" is kind of hard to determine when causes and effects blur together like that. <_<
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