Koopa Kid, also known as Baby Bowser and Mini-Bowser, is Bowser's henchman in the games Mario Party 2, Mario Party 3, Mario Party 4, Mario Party 5, Mario Party 6, and Mario Party 7. He is a playable character in Mario Party 5 and Mario Party 6.
Koopa Kid has never been seen after Mario Party 7, possibly in favor of Bowser Jr., since both characters look alike. Yasuyuki Oyagi, one of the directors of Mario Kart 7 has stated that Koopa Kid is retired, much to some fans' disappointment.
Koopa Kid only appears on the final board Eternal Star, where players have to pay 20 coins and win a dice block mini-game to obtain a Star. After all seven stars are collected seven new Star spaces will reappear.
Mario Party 2
Koopa Kid (called Baby Bowser) takes the place of the star once a player has taken it. If you land on his space he takes five coins. If a character wins a Bowser Bomb, the item will activate on its own and turns Baby Bowser into Bowser.
Mario Party 3
In this game, Koopa Kid now owns an Item Shop in the same location as the one owned by Toad on the board, with each turn randomizing whose shop will come up as a player passes. Koopa Kid's items are all the opposite of ones sold in Toad's store (Example: Toad sells Boo Repellent while Koopa Kid sells Boo Bells), except for the Item Bag and the Skeleton Key. Koopa Kid also appears in several of the Mini-Games, serving as the "losing prize" in the Item Mini-Games.
Mario Party 4
Koopa Kid makes a minor appearance in Mario Party 4 as the character who explains the rules of Bowser Mini-games and takes your coins if you land on a Bowser Space as well as announcing the total score in Darts of Doom. The other Koopa Kid appears fanning Bowser.
Mario Party 5
In Mario Party 5, the Koopa Kid is a playable character from the start. Three Koopa Kids, a green, a red and a blue one appear as opponents in Story Mode, which means that Koopa Kid can't be chosen in it.
Mario Party 6
In Mario Party 6, Koopa Kid appears as a playable character. The three Koopa Kids from Mario Party 5 make a return appearance in this game as opponents in the mini-games of Single player mode. The normal Koopa Kid is now a playable character in Single Player mode as well, and he seems to be on Mario's side rather than in Bowser's side.
Mario Party Advance
He makes his first appearance by squishing the Toad and hurl all Gaddgets away. It has its own role appearing from nowhere and sometimes play Rochambeau. If the player wins the player continues. He even appears on mini-game spaces. If the player wins, he is rewarded 1 Mushroom which Koopa Kid sulks about giving the mushroom. If the player loses, Koopa Kid takes away 1 Mushroom.
Mario Party 7
In Mario Party 7, he has a space named the Koopa Kid Space which charges players if they land on it. Three multi-colored Koopa Kids appear in Bowser Mini-games. This is also the last Mario Party game to have the Koopa Kids due to Bowser Jr. taking over the role.