The Skull Kid is a character in The Legend of Zelda series. His first appearance was in Ocarina of Time, and he also appears in Majora's Mask as the primary antagonist of the game. He also appears in Twilight Princess where he appears as a background character.
Ocarina of Time
In Ocarina of Time, three Skull Kids can be found within the Lost Woods. The first can be found through the first left-hand tunnel as Link enters the forest. If Young Link stands on the tree stump and pulls out his Ocarina to play Saria's Song, the Skull Kid gives Link a Piece of Heart and befriends him. After this, the Skull Kid purchases the Skull Mask from Link, but only for 10 Rupees, half of what it costs.
The two other Skull Kids offer Link to play a sing-along game. The Skull Kids play a tune and Link has to play back the same tune from memory; every time Link is successful, the Skull Kids add an additional note to the tune, reaching eight notes total. When Link recalls all eight notes from memory, the Skull Kids reward him with a Piece of Heart.
Skull Kids do not trust adults; when Link returns to the forest seven years later, the Skull Kids react negatively and attack him. If Link manages to defeat a Skull Kid, he is rewarded with 50 Rupees.
The Skull Kid in Majora's Mask is known to be the same Skull Kid for whom Link played Saria's Song in Ocarina of Time; this is implied at the end of the game when the Skull Kid says that Link has the same smell as the fairy child that taught him a song in the woods.
Skull Kid had become friends with the Four Giants that created the four lands of Termina: swamp to the south, mountains to the north, oceans to the west, and canyons to the east. When they finished, they started to leave, but the Skull Kid tried to stop them. After they left, he started causing trouble with two fairies: Tael and his sister Tatl. The Skull Kid desired power, so he stole Majora's Mask from the Happy Mask Salesman. Skull Kid became possessed by Majora's Mask and became its puppet, wreaking havoc among the natives of Termina and attempting to bring the Moon crashing on to the land destroying and killing every living thing. Link manages to stop his plans by calling the Four Giants, at which point Majora releases the Skull Kid from his grasp. After defeating Majora's Mask, Link and the Skull Kid become friends.
The Skull Kid first appears in Twilight Princess when Link enters the Lost Woods. The imp leads Link through the Lost Woods via a game of hide and seek. The game Skull Kid plays is fairly simple: there is always a clue as to where he is, as the orange glow from Skull Kid's lantern can often point Link in the direction he took off in. His horn, which resembles Deku Link's pipes in Majora's Mask, also guides Link to him. When Link gets near him, he hears him playing the familiar Saria's Song, which sets the mood perfectly. During the game of hide and seek, Skull Kid repeatedly summons Puppets to attack Link as he chases him, while Skull Kid never attacks Link directly.
When Link returns later in the game, Skull Kid once again helps Link navigate the Sacred Grove, although this time, he leads Link to the entrance of the Temple of Time. This time around, his hiding place is harder to find. Two things give away his location: the light of his lantern and the sound of his instrument playing Saria's Song. Again, the Skull Kid never physically attacks Link, though he summons Puppets to do the job. Every time he is hit in battle, he summons more Puppets, but he can be beaten quickly with the bow and arrows. After defeating him the second time, he tells Link that it was fun and then disappears.
Super Smash Bros. for Nintendo 3DS and Wii U
Skull Kid makes an appearance in Super Smash Bros. for Nintendo 3DS and Wii U as an Assist Trophy. He is wearing Majora's Mask. When summoned by a player, he casts one of three effects on the battlefield. One which flips the screen upside-down, inverting all of Links' movements; one which causes all fighters to momentarily be given the "clear" effect, making them difficult to see; and one which reverses directional inputs.