Bowser

Bowser is the main antagonist of the Super Mario Bros. series and Mario's arch-nemesis. He will star in the upcoming 59th episode of Death Battle Bowser VS Ganondorf, where he will fight Ganondorf from The Legend of Zelda.

History
King Bowser Koopa is the leader of the Koopa pack who has been an archenemy of Mario. Though Bowser most commonly kidnaps Princess Peach to get control of the Mushroom Kingdom, Bowser has joined forces with Mario in rare cases when they have a common enemy.

One Minute Melee
Bowser appeared in the 8th episode of One Minute Melee where he was pitted against Dr. Eggman and won.

Bowser also makes a cameo after the Bowser Jr. vs. Metal Sonic One Minute Melee, where he witness his son, Bowser Jr. getting defeated by Mecha Sonic. The scene ends with Bowser staring at Mecha Sonic, indicating that the two are going to fight.