Koopa Kid, formerly known as Baby Bowser, is a character who made his debut in Mario Party. He appeared in every Mario Party game until Mario Party 7.
Koopa Kid is a loyal minion of Bowser. He was known as Mini-Bowser in PAL territories, Mini Koopa in Japan and Baby Bowser in Mario Parties 1, 2, and 3. Koopa Kid resembles a miniature version of Bowser. In the earlier games in the series, there appears to be an arbitrarily large numbers of Koopa Kids, as shown by minigames such as the Mario Party 3 mini-game Baby Bowser Bonkers. Indeed, Mario Party 4 specifically refers to the Koopa Kid who appears on Bowser Spaces as "one of the Koopa Kids". However, starting in Mario Party 5, there seems to be only a single Koopa Kid, likely due to his new playable status. However, more than one can appear at once in Mario Party 7 (where Koopa Kid is non-playable again) in Pyramid Park.
In the Mario Party series
Baby Bowser made his first appearance in this game. However, he only appears in Eternal Star, the final board of this game. He replaced Koopa Troopa as the host, and he also replaced Toad in order for the players to challenge him. Each of the seven Baby Bowsers is stationed in the different location on the game board, and can be challenged by any player for 20 Coins. If the player pays 20 Coins, the players will have to play a roulette game against Baby Bowser. Both will hit the blocks, and the character with a highest number wins a Star. The players get their advantages because their dice block numbers are 8-10. If the player wins, they get Baby Bowser's Star and Baby Bowser disappears from the board. However, if Baby Bowser wins (which is unlikely), then he takes a Star from the player. When all seven Baby Bowsers are defeated, another seven Baby Bowsers appear and the player must start all over again. This continues until the game is completed. In the board's ending cutscene, the winning player uses the Stars to combine them into a large star, and sends both Bowser and Baby Bowser flying into outer space.
Mario Party 2
Baby Bowser appears in this game, and his role expanded slightly. In every board, a single Baby Bowser appears, in a single capacity to Toad, who sells Stars. Baby Bowser is initially placed in one of several predetermined locations on the board alongside Toad, and every time that a Star is bought from Toad, he will move to Toad's previous location. If a player passes on Baby Bowser, he will steal five Coins from that player. However, he occasionally gives the player five Coins instead, later claiming that this was an accident. Baby Bowser will always give the player Coins if he or she is wearing a Bowser Suit. If a Bowser Bomb is collected by any player in an Item Mini-Game, Baby Bowser will transform into Bowser at the end of the turn, who will hit three blocks and travel around the board, stealing all the Coins from any player he passes. This also occurs if someone lands on a Bowser Space and a roulette lands on "Bowser's Appearing Act". Once the player completes the five boards, this unlocks the final board of the game, Bowser Land, where Bowser and several Baby Bowsers kidnap Toad and one Baby Bowser replaces him. Several Baby Bowsers appear on Bowser Land, including the one Baby Bowser who hosts the board instead of Toad. Another Baby Bowser hosts the Bowser Parade, allowing the player to change the direction of the parade for five Coins (or for free if the player uses a Bowser Suit). Bowser Land's "Bowser Banks" are run by an unique blue, glass-wearing Baby Bowser Banker. The secondary Item Shop on the board is also run by a red, monocle-wearing Baby Bowser Shopkeeper, who will forcibly sell players Items for more Coins than usual.
Mario Party 3
Baby Bowser appears once again in this game. In every Party Board, Baby Bowser ran his own Item Shop, which sometimes appears in place of Toad's, where he sold sneaky Items such as Lucky Lamps, Reverse Mushrooms, and Boo Bells. Baby Bowsers also appear in every Item Mini-Games, where they often symbolize losing, such as the Baby Bowser space in Winner's Wheel. In Duel Mode, Baby Bowser also appears as one of the possible partners. He is not a default partner, but he can be obtained through the lottery. His attack is one, and his health is one. He can occasionally turn into Bowser and deals triple damage when attacking; however, if the transformation fails, he deals no damage. His salary is three Coins per turn. In Story Mode, Baby Bowser hears about the Star Stamps and reports to Bowser. Two Baby Bowsers appear, either with Bowser or without him, for every time the player wins a Star Stamp. When the player disappears after winning a Wit Star Stamp, the Baby Bowsers tells Bowser that the player disappeared, therefore missing their chance to steal the Star Stamps. They appeared again, and they told him that the player, who won the Kindness Star Stamp left again, missing their chance again. After the player wins the Strength Star Stamp, Bowser and Baby Bowsers finally appeared, but the player ignores them and they went after Bowser who disappeared after being ignored. They once again appeared, this time surrounding the player who won the Love Star Stamp, until Tumble told Bowser to step aside as Bowser lets the player go. The Baby Bowsers appeared seeing Bowser from far away in the sky after Daisy slapped him. They appeared one last time when they saw Bowser being knocked unconscious by Waluigi and they carried him away, never to be seen again for the rest of Story Mode.