Skull Kid is a character that appears in the Legend of Zelda series of games, having been controlled by Majora's Mask, wreaking havoc amongst the townspeople of Termina, and trying to destroy the world by smashing down the moon onto earth.
Skull Kid makes his debut as an Assist Trophy. When he is summoned, it can trigger a number of different effects such as flipping controls, turning all the fighters invisible, blacking out the screen, or flipping the stage (horizontally or vertically).
It's said that children who get lost in the woods 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!