Yen Sid

Yen Sid is a powerful sorcerer and teacher of Mickey Mouse, who first appeared in The Sorcerer's Apprentice, a segment of Fantasia. In Kingdom Hearts II, his voice is provided by Corey Burton.

Personality
Yen Sid is extremely wise and a very strict person. He is best known for his wisdom and skills with magic. Although Yen Sid is strict, he seems to have a sense of humor. Yen Sid cares deeply of Mickey's goal in becoming a great sorcerer and Yen Sid does whatever it takes to make Mickey's dreams come true.

Fantasia
The great sorcerer first appeared in the Fantasia short, The Sorcerer's Apprentice, as Mickey Mouse's master. He was the owner of a magical hat, which is apparently the source of his power. In the short, Mickey steals the hat and begins to play with its magic. But, as he was warned by Yen Sid, the power of the magic cannot be controlled by Mickey and wreaks havoc. Yen Sid arrives and manages to stop the magical chaos with his own powers, proving his mastery of magic is still powerful even without his hat.

Fantasia 2000
Yen Sid reappears in the sequel and lives out the same role in the original. His sequence in the Fantasia sequel being the only sequel from the original film to appear in the film.

House of Mouse
In the animated television series House of Mouse, Yen Sid makes a few cameos, usually alongside the magic brooms from the Fantasia short.

Kingdom Hearts series
Majority of information taken from Kingdom Hearts Wiki-Yen Sid.

Yen Sid appears in Kingdom Hearts II and Kingdom Hearts Birth by Sleep 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.

Kingdom Hearts Birth by Sleep
After training Mickey on the night before Terra and Aqua's Mark of Mastery exam, Yen Sid relays to him a, "feeling that something terrible is about to transpire." The next morning, following the completion of the exam, Yen Sid contacts Master Eraqus via the crystal in the Land of Departure's throne room about the threat of the Unversed.

When Terra visits Yen Sid's tower, he asks him for guidance about Master Xehanort, Vanitas, and the Unversed. After Terra leaves, Yen Sid comments "I had hope Xehanort, your heart would no longer lead you astray". Later, Ventus shows up at his tower with a Star Shard, and Master Yen Sid uses divination magic to locate Mickey. Aqua later returns Mickey to his tower, and Master Yen Sid tells her of Master Eraqus's destruction and that the murder was committed by Master Xehanort and Terra. Sometime after Aqua leaves for the Keyblade Graveyard, Mickey departs to assist her and her friends.

Following the defeat of Vanitas, Mickey returns with Aqua and a comatose Ventus. Master Yen Sid finds himself unable to sense Ven's heart and encourages Aqua to believe in their friendship—which serves as the key to bring him home one day. Some time later, Mickey sorrowfully hands in the Star Shard and the Star Seeker, thinking he has disgraced his master, and prepares to depart. However, Master Yen Sid gives him back the Keyblade, and he happily accepts.

Kingdom Hearts II
Eleven years later in Kingdom Hearts II, Yen Sid serves as Sora's tutor to what lies ahead of him in his journey. He lives in a large tower in a small world-of-sorts connected to Twilight Town via a Magical Train. He dwells at its top, where he studies the Heartless, the Nobodies and Organization XIII. When Sora, Donald Duck, and Goofy arrive, he tells them of the enemies that they will face in the near future (Nobodies, Heartless and the Organization). He also returns to them their Gummi Ship and the co-ordinates to Hollow Bastion.

It is unknown what happens to Master Yen Sid and the 3 fairies, Flora, Fauna, and Merryweather. The last we see of Master Yen Sid is after the Gummi ship is returned to Sora, Donald, and Goofy, and he vanishes. The last seen of the three fairies is when Maleficent's cloak is brought to the tower and they decide to tell Yen Sid about this. Later on if you visit the tower, it is overtaken by Nobodies (more specifically, Sniper, Dragoon, Berserker, Dancer, Gambler, and Dusk), and neither Yen Sid or the fairies are anywhere to be seen. It is assumed, however, that he escaped to Traverse Town, Disney Castle or Hollow Bastion, though most likely in an inaccessible area in the later two worlds, while the fairies either returned to Aurora's world or followed Master Yen Sid to wherever he escaped.

Kingdom Hearts coded
After discovering that Sora has the power to save all the lost hearts connected to him, Mickey meets with Yen Sid to inform him of the whereabouts of Ventus's heart, leaving only Terra to be found. However, Yen Sid shocks Mickey by revealing that the destruction of Ansem and Xemnas will lead to the return of Xehanort, their original persona. Fearing that Xehanort may not be alone, Yen Sid orders Mickey to bring Sora and Riku to the Mysterious Tower to undertake the Mark of Mastery exam.

Epic Mickey
Years ago, Yen Sid created a home for the forgotten and retired aspects of Disney, serving as a benevolent, albeit unseen creator, watching from "on high" and enjoying their antics. But one day, his magic mirror opened into the home of Mickey Mouse. When he entered Yen Sid's workshop after he was resting from that evening's work, Mickey played with the magic paintbrush and created a monster, ruining Yen Sid's creation in the process, now dubbed the Wasteland.

In the end, Mickey returns home and is apparently locked out of Yen Sid's studies. Although it seems to be impossible to return, Yen Sid remembers he is Mickey Mouse and with that personality will easily find a way to return.

Trivia

 * Yen Sid's name, which is not given in the film itself, was given to him by the animators. It is "Disney" spelled backwards.
 * Kingdom Hearts II marks the first time Yen Sid's voice has ever been heard.