Sora/Venitus - Haley Joel Osment: Also played the kid from The Sixth Sense.
Roxas/Ventus - Jesse McCartney: Famous for his singing career.
Kairi - Hayden Panettiere: Also played Claire from Heroes, amongst other roles.
Ansem the Wise - Christopher Lee: Also played Count Duku from Star Wars.
Aqua - Willa Holland: Also starred as Kaitlin Cooper from The OC and Audrey from Legion.
Master Eraqus - Mark Hamill: Also known as Luke Skywalker, and the voice of Joker.
Xehanort - Leonard Nimoy: Also played Spock from Star Trek and William Bell in Fringe