Yen Sid

Yen Sid is a powerful sorcerer who made his Disney debut in Fantasia. He 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. His name is an of "Disney", as a tribute to  himself.

Kingdom Hearts II
The Sorcerer's Apprentice (1940)

''A powerful sorcerer, second to none. He tutored the King in magic.''

''From conjuring up skies full of beautiful butterflies to parting an ocean, Yen Sid's feats of wizardry are...well, amazing. His name alone fills some folks with awe.''

''Yen Sid shared some of his wisdom with Sora and his friends. He also taught them some valuable techniques to help prepare them for what they'll face on this new quest.''

Kingdom Hearts Birth by Sleep
The Sorcerer's Apprentice (1940)

''A great sorcerer who dwells within the Mysterious Tower. Mickey is his apprentice.''

Yen Sid was the first to detect something amiss with the worlds, and alerted Master Eraqus as soon as he could.

He is concerned about his apprentice, Mickey, who has run off.

It was also he who delivered to Aqua the news of Master Eraqus's demise at the hands of Terra and Master Xehanort.

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 hoped, Xehanort, that 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 Terra and Master Xehanort. Sometime after Aqua leaves for the Keyblade Graveyard, Mickey departs to assist her and her friends.

Following Vanitas's defeat, 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 his Keyblade, thinking he has failed his Mark of Mastery Exam and disgraced his master, and prepares to depart. However, Master Yen Sid gives him back the Keyblade, showing that Mickey, in fact, passed and became a Master.

Kingdom Hearts II
Yen Sid serves as Sora's tutor to what lies ahead of him in his journey. When Sora and company (Donald Duck and Goofy) arrive, he tells them of the enemies that they will face in the near future (Heartless, Nobodies, and Organization XIII). He also returns their Gummi Ship to them, which he had been safekeeping, and sends them to Hollow Bastion.

When Diablo brings Maleficent's cloak to Flora, Fauna, and Merryweather, causing her to be revived, the fairies rush off to tell Yen Sid. However, by the time Sora and his friends can return to the tower, Yen Sid and the fairies are nowhere to be found.

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 discuss their ongoing search for Terra, Ventus, and Aqua. However, Yen Sid shocks Mickey by revealing that the destruction of Ansem and Xemnas has led to the return of Xehanort, and not just a "single one" of him, and which will eventually lead to the return of Master Xehanort. Yen Sid had known that this disaster might come about while he was mentoring Sora, but chose not to disclose it to anyone because leaving Xemnas alive was not an option. As Mickey, the sole surviving Keyblade Master, will not be enough to counter Xehanort's new threat, Yen Sid orders Mickey to summon Sora and Riku to him to undertake the Mark of Mastery Exam.

Kingdom Hearts 3D: Dream Drop Distance
In the 2011 TGS trailer, Yen Sid summons Sora and Riku before him in the Mysterious Tower, telling them to unlock the keyhole to the "world of sleep". Only by returning from the "world of sleep" he will consider both Sora and Riku to be Keyblade Masters. In the February 2012 special trailer, he and Mickey discuss the possibility of the two of them aiding Sora and Riku in their mission, and in a later scene, he appears to be asking a favor from Lea.

Appearance
Master Yen Sid is a rather severe-looking old man with long, grey hair, bushy eyebrows, and a long, grey beard that parts down the middle. He wears the famous, conical sorcerer's hat on his head. The hat is blue and decorated with several yellow stars and a yellow crescent moon. Fantasia shows that the top of Yen Sid's head is bald under the hat, though he never removes it in the Kingdom Hearts series. He wears long, billowing blue robes with sky blue lining. The insides of his robes are dark red. The whites of his eyes are quite large in comparison to his small pupils, something that is further highlighted by the fact that his eyes are often wide open. In Kingdom Hearts Birth By Sleep, there are very small changes to the sorcerer's robes, such as the sky blue lining becoming thicker and the robes themselves being a bit wider.

Personality
While little is known about his life as a Keyblade master, the elderly sorcerer is shown to be a rather stern man. In almost every instance, he is shown to be frowning. But beneath his harsh exterior belies a man who strongly believes in his apprentices no matter how much mischief they may get up to, demonstrated by when he expresses annoyance at Mickey for running off with the Star Shard and recklessly getting lost with it, but then forgives Mickey and awards him the title of Master with an encouraging smile.

Given his past status as a Keyblade master, he is shown to have great knowledge about the properties of the Heart, the Heartless, the Nobodies and the Unversed. It is he who explains the nature of the Unversed to Master Eraqus, and that of the Nobodies to Sora, Donald and Goofy, and also tells them that as long as darkness exists in one heart, the Heartless will never cease to exist. So powerful and wise he is, that even Terra, Aqua, and Ventus speak to him with utmost respect, despite him not being their master. However, Mickey merely refers to him as "Yen Sid" without calling him "Master", possibly because Mickey, like Master Eraqus before him, is also a Keyblade Master.

Abilities
Master Yen Sid is a master of the magical arts, able to perform powerful magic spells comparable to Merlin's. He has shown the ability to teleport, create illusions while training Mickey, and create cosmic smoke to locate a person. He seeks counsel with the stars to predict the conditions of the present and the future, which allows him to gather information about the worlds' condition and to pass it on to his allies.

As a retired Keyblade Master, he presumably stills retains the power to use the Keyblade, but never used it since he had taken his leave.

Origin
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 he wears in Kingdom Hearts II) that is apparently the source of his power. In the movie's most acclaimed sequence, 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.

Trivia

 * Yen Sid's English voice actor, Corey Burton, also voices Ansem the Wise in Re:Chain of Memories and Birth by Sleep. In addition, Corey Burton also voices Sark, the Magic Mirror, the White Rabbit, the MCP, Dale, Captain Hook, Flotsam and Jetsam, and Shan Yu throughout the series of games.