The Koopa Kids are minions of Bowser who only appear in the Mario Party series. They are known as Mini-Bowsers in PAL territories, Mini Koopa in Japan (Bowser's name in Japan is Koopa) and Baby Bowsers in Mario Parties 1, 2, and 3. The Koopa Kids resemble miniature versions of Bowser (and are therefore very similar in appearance to Bowser Jr. and Baby Bowser). A Koopa Kid also appears as a playable character in Mario Party 5 and Mario Party 6.
In the earlier games in the series, there appears to be an arbitrarily large number of Koopa Kids, as shown by mini games 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.
Mario Party 5 also introduced Green K. Kid, Red K. Kid and Blue K. Kid (Green M. Bowser, Red M. Bowser and Blue M. Bowser in PAL versions) who, as their names imply, are colored differently. In Mario Party 5, their first appearance, they are shown to be the result of one Koopa Kid splitting into three.
The Koopa Kids made their debut appearance in Mario Party, as Bowser's henchmen. They are only seen on the final board of the game, Eternal Star.
After playing Bowser's Magma Mountain, Bowser will steal all the Stars that the player collected throughout the game and travel to Eternal Star. When the players find him, he forces them to challenge seven Koopa Kid to get the Stars back.
Each of the Koopa Kids is stationed in a different location on the game board, and can be challenged by any player for 20 coins. If this is done, the player will have to play a short minigame against the Koopa Kid. Both characters will hit a Dice Block, with the higher number winning. It is easy for the player to win, as the player's is rigged to only roll 8-10. If the player wins, he or she gets the Koopa Kid's Star and the Koopa Kid disappears from the board. However, if the Koopa Kid wins (which, as mentioned before, is unlikely), the Koopa Kid will take one of the player's Stars.
When all seven Koopa Kids are defeated, a new group appears 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 send the Koopa Kids and Bowser flying into space.
Mario Party 2
In Mario Party 2, a single Koopa Kid appears on every board in the game, in a similar capacity to Toad, who sells Stars. He is initially placed in one of several predetermined locations on the board alongside Toad, and every time that the Star is bought from Toad, he will move to Toad's previous location. If a player passes Koopa Kid, he will steal 5 Coins from that player. However, he occasionally gives a player 5 Coins instead, later claiming that this was an accident. He will always give the player coins if he or she is using a Bowser Suit.
If a Bowser Bomb is collected by any player in an Item Mini-Game, Koopa Kid will briefly be replaced by Bowser at the end of the term, who will hit three Dice Blocks and then travel around the board, stealing all coins from any player he passes.
Several Koopa Kids appear in Bowser Land, including one who hosts the board instead of Toad. Another Koopa Kid hosts the Bowser Parade. Bowser Land's Bowser Banks are run by a unique blue, glasses-wearing Koopa Kid Banker. The secondary Item Shop on the board is also run by a red, monocle-wearing Koopa Kid Shopkeeper, who will forcibly sell players items for more Coins than usual.
Mario Party 3
In Mario Party 3, Koopa Kid 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. Koopa Kids also appear in the Item Minigame, where they often symbolize losing, such as the Koopa Kid space in Winner's Wheel.
In Duel Mode, Koopa Kid also appears as one of the possible partners. He isn't a default partner, but can be obtained through the lottery. His attack is one, and his health is one. He can occasionally turn into Bowser and deal triple damage when attacking; however, if the transformation fails, he deals no damage. Koopa Kid's salary is three coins per turn.
Mario Party 4
In Mario Party 4, a Koopa Kid would often come in place of Bowser whenever someone lands on a Bowser Space, and would steal 10 to 30 Coins from either the player who landed there, or from all the players. Sometimes, the Koopa Kid would then call Bowser after stealing coins, and a Bowser event would begin. In the Bowser Minigames, two Koopa Kids work as Bowser's servants; one of which is seen fanning him and the other one explains the rules to the game (which may be Fruits of Doom, Darts of Doom or Balloon of Doom) and in addition, he also brings the dart board to the players and announces the scores in Darts of Doom. The same two Koopa Kids appear in The Final Battle!, attacking the player with fireballs from Koopa Clown Car-like vehicles while the player solves a puzzle. One Koopa Kid also hosts the last board: Bowser's Gnarly Party and is seen selling items and hosting board games. Unlike other board hosts, this Koopa Kid appears identical to others. In addition, Koopa Kid can be unlocked as a playable character in Beach Volley Folley.
Mario Party 5
In Mario Party 5, Koopa Kid is a playable character, except in Story Mode and Card Party. In Story Mode, Koopa Kid splits up into Red K. Kid, Blue K. Kid and Green K. Kid, and the player has to steal all their coins on abridged versions of the regular boards to defeat them. Unlike in a normal game, all three Koopa Kids move at the same time. In Card Party, Koopa Kid shows up to cause trouble on the Bowser Card.
Mario Party 6
In Mario Party 6, Koopa Kid is again a playable character. The colored Koopa Kid trio also reappears as the player's opponents in Solo Mode's mini games. Unlike in Mario Party 5, Koopa Kid is playable in all game modes.
Mario Party 7
In Mario Party 7, Koopa Kid is no longer playable. Instead, he appears whenever someone lands on a Koopa Kid Space, and will do things such as swapping players' coins or positions, or use a Cursed Mushroom on the player/team who lands on his space. Red K. Kid, Blue K. Kid and Green K. Kid were also the main antagonists in the three multiplayer Bowser minigames, and they are seen operating the cannons in Bowser's Enchanted Inferno!. A Koopa Kid is also seen as one of the hazards (alongside Bowser) in Bowser's Lovely Lift!.
Mario Party Advance
In Mario Party Advance, Koopa Kids could be seen working with Bowser, and appear in some of his minigames, where the object was usually for the player to defeat them. Up to ninety-nine of them could appear at once. One main Koopa Kid appears throughout the game as Bowser's right-hand man, often appearing to antagonize the player by doing things such as stealing Mushrooms from them or flinging them somewhere else in Shroom City.
Relationship with Bowser
In the first Mario Party Bowser introduces Koopa Kid, then known as Baby Bowser, as his "cohort."
In Mario Party 4, whenever a Koopa Kid calls for Bowser, he refers to him as "Dad". However, it is unlikely that Bowser is the biological father of all the Koopa Kids, and that the Koopa Kids may just be clones of some nature. Bowser's only child is Bowser Jr. (or, in earlier materials, the Koopalings). Similarly, Bowser also grounds them by taking away their Mario action figure if they fail him, hinting that Bowser at the very least has a father-like relationship with them. In Mario Party 7, the Koopa Kids appear to work for Bowser for money, calling him "Mr. Bowser" and expecting to get a raise from Bowser after causing trouble on a Koopa Kid Space.