Who are the important characters of Enders's game?

Well three main characters are Ender (obviously) and his siblings, Peter and Val.

As for a fourth main character, it's my opinion that it's up for discussion. I'd probably say Bean myself. But while Bean is a large part of the latter half of Battle School, Alai is a pretty large part of the first half - so you could say Alai. And as a third possibility, you could say Col. Graff - he has a sort of overarching presence throughout the book.