Skull Kids (スタルキッド, Stal Kid) are recurring characters in The Legend of Zelda series. They appear in Super Smash Bros. Brawl as stickers. The one from The Legend of Zelda: Majora's Mask appears in Super Smash Bros. 4, wearing the titular mask, as an Assist Trophy.
Skull Kids first appeared in The Legend of Zelda: Ocarina of Time as minor characters, but one returns (specifically, the one who befriends Link once he plays Saria's Song to him) in Majora's Mask as the main antagonist in the game under the control of Majora's Mask. This Skull Kid is the basis for Super Smash Bros. 4's Assist Trophy. A Skull Kid (thought to be the same one as he knows Saria's Song) also appears as a minor character in The Legend of Zelda: Twilight Princess as a miniboss.
The Skull Kid's attack as an Assist Trophy is not based on any known ability of Majora's Mask itself, although Skull Kid has been seen using the power of the mask to curse people for his own amusement. Two of his attacks, however, have a mechanic that makes reference to the Stone Tower and its respective temple, which must be inverted for Link to progress.
In Super Smash Bros. Melee
In Super Smash Bros. Brawl
Skull Kids appear on two stickers in Brawl.
In Super Smash Bros. 4
As an Assist Trophy
The Skull Kid from The Legend of Zelda: Majora's Mask appears in Super Smash Bros. 4 as an Assist Trophy. As one of the Tips mentions, Skull Kid is one of the only Assist Trophies to have potentially detrimental effects to both enemies and the user. When he appears, he can do one of three things, chosen at random:
It's said that children who get lost in the forest are turned into these music-loving little monsters. In Smash Bros., you can summon one who's sporting Majora's Mask. It'll move into the background and bring down a world of chaos on the whole stage. Even the fighter who summoned him will be affected, so try to keep a level head!
It's said that children who get lost in the forest are turned into these music-loving little monsters. You can summon one in Super Smash Bros. Wearing Majora's Mask, it'll move into the background and bring down a world of chaos on the whole stage. Even the fighter who summoned him will be affected, so try to keep a level head!
In Super Smash Bros. Ultimate
As an Assist Trophy
Skull Kid once again returns as an Assist Trophy from Super Smash Bros. 4. Training Mode describes his Assist Trophy: Does disruptive things like turning the stage upside down or making fighters invisible. Can't be KO'd. When playing on the Spear Pillar stage, Skull Kid cannot be summoned.
As a costume
Skull Kid is also the basis for a costume for Mii Brawlers.