Skull Kid

Skull Kids (, Stal Kid) are recurring characters in series. They appear in Super Smash Bros. Brawl as stickers. The one from  appears in Super Smash Bros. 4, wearing the titular mask, as an Assist Trophy.

Origin
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  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 and its respective temple, which must be inverted for Link to progress.

In Super Smash Bros. Melee
Skull Kid himself did not appear, but he was mentioned in the description of the trophy and the  trophy.

In Super Smash Bros. Brawl
Skull Kids appear on two stickers in Brawl.

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:
 * Flips the camera upside-down, as well as mirroring it.
 * Reverses all directional inputs for all players, so left is right and up is down. This does not apply to taunts.
 * Turns all fighters invisible.

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 which is likely due to Palkia which can affect the screen most likely similar to what Skull Kid usually does as an Assist Trophy. Along with Spear Pillar, Skull Kid cannot be summoned on 3D Land, Arena Ferox, Dream Land, Hyrule Castle, Jungle Japes, Magicant, Mario Circuit, Midgar, Peach's Castle, Reset Bomb Forest, Skyloft, Super Mario Maker, Tortimer Island, Umbra Clock Tower, and WarioWare, Inc.

As a costume
Skull Kid serves as the basis for the Skull Kid's Hat Headgear and the Skull Kid's Outfit for the.

Spirit
Skull Kid also appears as two different primary spirits.

In Adventure Mode: World of Light, Skull Kid is presumably one of the countless spirits captured by Galeem during his takeover of the universe. After Galeem's defeat, Dharkon takes control of Galeem's spirits, including Skull Kid. Skull Kid's spirit is located in the Sacred Land sub-dungeon of The Dark Realm.

Skull Kid's Spirit Battle uses a Mii Brawler puppet fighter wearing a Skull Kid's Hat and Outfit and can be fought on the Distant Planet stage. He is fought along two Tiny puppet fighters representing Skull Kid's fairy companions, Tatl and Tael. During the battle, Skull Kid appears as a hostile Assist Trophy while the stage is covered in fog, referencing the fog of the Lost Woods where the Skull Kid resides.

Skull Kid & Majora's Mask can be obtained by enhancing the Skull Kid spirit at level 99.

Trivia

 * Usually, when Skull Kid makes the fighters invisible, they stay invisible upon losing a life. However, if this happens in Smash Tour, the newly-respawned character will become visible.
 * If Skull Kid flips the screen in Training Mode, the training menu and info will also be flipped.
 * Conversely, if a character is out of the camera's view and taking hoop damage during a standard match, the hoop will point at the character's actual location, instead of following the flipped camera view.
 * Skull Kid is one of the few Assist Trophies to be revealed on the Super Smash Bros. Ultimate website rather than in a Nintendo Direct.
 * Skull Kid cannot be summoned on Spear Pillar due to his abilities conflicting with those of Dialga and Palkia. For similar reasons, Palkia cannot be summoned from a Poké Ball alongside Skull Kid and vice versa.

Skull Kid