Yen Sid

Master Yen Sid is a powerful sorcerer who made his Disney debut in Fantasia. He appears in Kingdom Hearts II as an aid to Sora and the gang. Yen Sid is well-known for his wisdom, and his command over the arts of sorcery is such that most address him as "Master". He was once Mickey Mouse's tutor, and taught him everything he knows of the arts of sorcery. His name is two separated parts of the name 'Disney' backwards as a tribute to Walt Disney himself. Yen Sid was also one of Walt Disney's alternate names.

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
Master Yen Sid first appears as Mickey is seen wielding the Star Seeker Keyblade. Mickey was trained under Master Yen Sid.

Through Master Eraqus's statement, it is revealed that Yen Sid is a retired Keyblade master and therefore possesses high knowledge about the Keyblade. He is seen to be an old friend of Master Eraqus and knows who Master Xehanort is.

When Terra visited his tower, he asked for guidance about Master Xehanort, Vanitas, and the Unversed. After Terra leaves, Yen Sid comments "Are you still plagued with strange thoughts, Xehanort?". Later, Ventus shows up at his tower with a Star Fragment 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 death 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.

By the end of the game, 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.

During the ending credits, Mickey sorrowfully hands in the Star Fragment and his Star Seeker Keyblade, 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, during the events of 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 their Gummi Ship to them and inputs the co-ordinates to Hollow Bastion.

It is unknown what happens to Master Yen Sid and the three 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.

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.

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. 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 Fragment and recklessly getting lost with it, but then forgives Mickey and encourages him to continue in his journey to become a Keyblade master.

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 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.

Trivia

 * In Kingdom Hearts Birth by Sleep, Master Eraqus states that Yen Sid is a retired Keyblade master. This makes him and Mickey the only Disney characters to be a Keyblade wielder.
 * In Kingdom Hearts Birth by Sleep, Master Yen Sid wonders if Master Xehanort still has "distracting thoughts". This is a sign that the two have possibly met before.
 * Kingdom Hearts II marks Yen Sid's first instance of speaking.
 * "Yen Sid" is "Disney" in reverse, similarly to how "Eraqus" is an anagram of "Square" referring to Square Enix.

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