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 "Disney" spelled backwards, as a tribute to Walt Disney himself.
Before Kingdom Hearts Birth by Sleep
Yen Sid was once a Keyblade Master alongside Eraqus and Xehanort. He chose to retire to the Mysterious Tower and lay down his Keyblade Master mantle, but remained a powerful sorcerer and continued to observe the balance between Light and Darkness.
Yen Sid took on the boatman Mickey Mouse as his apprentice, teaching him everything he knows about magic and the Keyblade. At one point, he sent Mickey on a mission to the Country of the Musketeers.
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 his feelings that a tragedy is soon to come. The next morning, following the completion of the exam, Yen Sid contacts 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 muses on his disappointment in Xehanort's continuing folly. 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
Yen Sid explains the full history of Master Xehanort to Sora and Riku and puts them through the Mark of Mastery exam by assigning them to awaken several worlds which remained in a state of "deep sleep" following Ansem's destruction, and explains numerous facts such as the nature of the Dream Eaters and residents of the sleeping worlds. However, soon after the exam begins, Yen Sid starts to feel that something is going awry.
Later on, when Mickey, Donald and Goofy depart for Disney Castle to confront Maleficent, Yen Sid surmises that the growing darkness in the worlds due to Master Xehanort's coming return may be the reason Maleficent has become strong enough to resist the Cornerstone of Light. The trio soon return with the revived Lea, who requests to become a Keyblade wielder. Yen Sid, taken aback, warns Lea of the dangerous path ahead, and has him undergo a magically compressed training session under Merlin and the three fairies. Eventually, when Sora and Riku do not return from the Sleeping Worlds as planned, Yen Sid realizes that Xehanort devised a means to confront the two in the Sleeping Worlds and sprung a trap on them. He allows Mickey to make his way to The World That Never Was to save their friends, sending Lea, Donald and Goofy after Mickey as back-up.
Mickey and the others eventually bring Sora's comatose body back to the study, where Yen Sid realizes that Master Xehanort was on to them the entire time. When Riku awakens Sora from his slumber, Yen Sid delivers a sincere apology to the both of them for failing to realize the magnitude of Master Xehanort's plans. Though he commends them both for performing admirably, Yen Sid only declares Riku as a Keyblade Master, since he managed to unlock two Keyholes of Sleep, while Sora failed to resist the darkness and ended up in a magical sleep induced by Young Xehanort. Yen Sid, however, is visibly stunned when Lea finally manages to summon his own Keyblade.
Having realized Master Xehanort's plan to recreate the χ-blade by pitting his new Organization XIII against the seven Princesses of Heart, Yen Sid devises a plan to counter Xehanort by gathering seven Keyblade wielders of Light to protect the Princesses, and orders Riku to bring Kairi to him to undergo training with the Keyblade she received from Aqua.
Kingdom Hearts 0.2 Birth By Sleep -A fragmentary passage-
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, with the insides of his robes being 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.
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, the Unversed, and the Dream Eaters. It is he who explains the nature of the Unversed to Master Eraqus, and that of the Nobodies and the Dream Eaters 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 is he, that even Terra, Aqua, and Ventus speak to him with utmost respect, despite him not being their master. However, Mickey and Riku merely refer to him as "Yen Sid" without calling him "Master", possibly because they, like Master Eraqus, are also Keyblade Masters (though this is only in the English versions; in the Japanese versions, they still call him "Master Yen Sid" as a sign of respect).
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 still retains the power to use the Keyblade, but does not appear to use it.
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. However, 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.