Well... There are not alot of characters that affect the future of Kingdom Hearts. Though, here are a few of them and what they did:
Terra:
Spoiler:Terra is just only one of the few that affect the games. Terra is a Character with strenghth but what happened to him? He became part of Xemnas (read more about it in Xenahort)
Ventus:
Spoiler:He looks exactly like Roxas cas he is Roxas. See Roxas is the center of Castle Oblivion. (Read more at aqua's section)
Aqua:
Spoiler:Aqua was named a Keyblade master and is the reason why Castle Oblivion is in King dom Hearts.
Master Xenahort:
Spoiler: He merged with Terra to have more power and a new body. After an experiment went wrong, he became Xemnas and started Organization IIX.