Kamek is a Magikoopa in Bowser's army. He debuted in Super Mario World 2: Yoshi's Island as Baby Bowser's caretaker and the game's main antagonist: throughout the game, as well as in later Yoshi games, he assists bosses with his magic, usually by making them gigantic.
Kamek later went on to appear in the New Super Mario Bros. series starting with New Super Mario Bros. Wii: once again he appears at the beginning of the boss battles to assist the boss, though unlike the Yoshi series in most cases his magic alters the arena instead of the boss itself.
Kamek appears as two stickers.
As a stage element
Kamek appears as a stage element on the Mushroom Kingdom U stage. When he appears, he casts a spell which switches the stage between its possible forms. He doesn't interact directly with the fighters.
Kamek is an enemy, and can fire beams which lower the player's stats.
In Smash Tour
Kamek appears as a random event in Smash Tour. He appears at the beginning of a turn and uses his magic to change the position of the checkpoints.
Kamek's trophy is part of the Yoshi's Island Trophy Box.
Kamek reprises his role in the Mushroom Kingdom U stage.
Kamek also appears as a spirit.