Master Yen Sid is a powerful sorcerer who made his Disney debut in Fantasia. He appears in Kingdom Hearts II, Kingdom Hearts Birth by Sleep, Kingdom Hearts coded and Kingdom Hearts 3D: Dream Drop Distance as a retired Keyblade Master, renowned for his wisdom and command over the arts of magic; though no longer an active Keyblade Master, he still keeps a close watch on the balance between Light and Darkness. He was once Mickey Mouse's master, and taught him everything he knows of magic and the Keyblade. His name is a reverse anagram of "Disney", as a tribute to Walt Disney himself.
Sora, you must focus on regaining your lost power of waking. Mickey and Riku, I recall that Master Aqua journeyed to many worlds and connected with others. In that case, rendezvous with Kairi and Lea, they are continuing their training under the tutelage of the wizard Merlin.