Yen Sid

Yen Sid, formerly known as Master Yen Sid, is a former wielder of the Keyblade. He's a powerful sorcerer who made his Disney debut in Fantasia. He once was the master of King Mickey, teaching him uses of the Keyblade and how to wield it. His name is an anagram of "Disney".

Kingdom Hearts: Light by Darkness
The Sorcerer's Apprentice (1940)

''A powerful sorcerer, who taught the King magic and Keyblade use. Once a Keyblade master, Yen Sid feats of wizardry is beyond expectations.''

''He gives quests out to Sora, and this time--not just to Sora, but to his friends as well. What is this quest for? It's more than just a simple quest, but also a pre-exam for something to come.''

Kingdom Hearts: Light by Darkness
He meets Sora, Riku, and Kairi at his chamber when they come. Yen Sid explains the concept of Vents to them, and the new threat that has surfaced--the Lightfeeders. Yen Sid gave this quest not only to help the worlds that are in peril, but also to test them to see if they are ready for the Mark of Mastery exam for Sora and Riku. His plans for Kairi, whatsoever, are different. He wants to see if she can manage the Keyblade, which will make use for in the future.