Skull Kid is a playable character in the fighting video game, M.U.G.E.N Trilogy. He was confirmed on November 2, 2017 along with several Nintendo characters. Sachi Matsumoto (who also voiced Toon Link) provides the role for Skull Kid in this fighting game.
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.
How to Unlock
Scan Skull Kid's AR Icon from the M.U.G.E.N Trilogy website.