Merlin appears to be calm and decisive. He has an active nature even though he seems to be one of the oldest people in the Kingdom Hearts series. Merlin is nice and seems to be caring to the ones close to him. However, he can be easily annoyed, especially when people insult his talent for magic, and he will not hesitate to physically retaliate.
Merlin is an elderly man with pale skin and thin arms and legs. His nose is a bit large and his black eyes are rather small and close-set. He wears spectacles with circular lenses. He seems to be balding and has short, white hair on the sides and back of his head. He also has bushy eyebrows, a walrus mustache, and a long, floor-length beard, all of which are white.
Merlin dresses in a sky blue cloak that stops just above his ankles and has large, hanging sleeves. He wears pointed slippers and a tall, floppy, conical hat, both of which are the same sky blue color as his cloak. Merlin's wand is a very long, featureless, brown rod with a small bump on one end.
Merlin appears in Radiant Garden before the world is engulfed in darkness. He accidentally drops the book granting access to the 100 Acre Wood, but Terra returns it to him. He also interacts with Aqua and Ventus while they visit the world. Looking inside the book grants the three Keybearers access to the 100 Acre Wood Command Board.
Merlin is sent by King Mickey to Traverse Town, where he sets up in the ramshackle Mystical House, along with the Fairy Godmother. Sora, Donald and Goofy can meet him as soon as they learn Fire, in order to open the door, but they must talk to him after Cid asks them to deliver the 100 Acre Wood novel to the wizard. Merlin appears in a puff of smoke to greet them, and teaches them how to properly use magic.
The upper level of the house is empty, but can be used for magical training against enchanted furniture. After Sora delivers the repaired book to Merlin, he explains its special nature, but Sora is magically sucked into the secret world. Merlin will also give the heroes the Spellbinder, Dream Rod, and Dream Shield once they reach certain levels of magical skill.
Merlin now lives in Hollow Bastion which is being rebuilt. He lives in a large cottage which serves as the headquarters for the Hollow Bastion Restoration Committee. Merlin is part of the committee along with Leon, Yuffie, Aerith, and Cid. Upon reuniting with Sora, he grants Sora the Blizzard element. Merlin gives Sora his first Summon charm which unlocks Chicken Little. Merlin also retrieves the 100 Acre Wood book from Traverse Town's Zero District and places it on a shelf in his house.
Later, he travels to Disney Castle with Sora, Donald, and Goofy to open up a magical door to the past. Merlin seems to have developed a rivalry with Cid, who prefers technology to magic, while Merlin prefers magic to technology. Their rivalry nearly gets out of hand when Cid calls Merlin an "old loon", and Merlin zaps Hollow Bastion's power computer, but thanks to Sora, Donald and Goofy, the town's systems are saved. Merlin and Cid later recall that Hollow Bastion was formerly called Radiant Garden and decide to rename it to its former name.
Merlin's first and only appearance outside of Kingdom Hearts (and the TV series, Disney's House of Mouse) was in the 1963 film, The Sword in the Stone. In the film, Merlin teaches the young King Arthur some life lessons and prepares him for a life as the ruler of England.