Koopa Kid is a playable character in the fighting video game, M.U.G.E.N Trilogy. He was confirmed on January 3, 2016.
The Koopa Kids (called Mini Bowsers in Europe, and originally known as Baby Bowsers in all non-Japanese versions of the first three Mario Party games) are minions of Bowser who only appear in the Mario Party series. 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 minigames such as Baby Bowser Bonkers in Mario Party 3, with the Koopa Kids being capable of splitting themselves. 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 of the regular coloration, likely due to his new playable status. The other Koopa Kids in the game are divided into color: Green K. Kid, Red K. Kid, and Blue K. Kid (Green M. Bowser, Red M. Bowser and Blue M. Bowser in British English). However, more than one standard Koopa Kid can appear at once in Mario Party 7 (where Koopa Kid is non-playable again) in Pyramid Park.
Koopa Kids have seemingly been removed from the Mario Party series starting in Mario Party 8, likely in favor of Bowser Jr., who appears in Mario Party DS and onwards. This is further suggested by Bowser Jr. outright replacing Koopa Kid in the Mario Party: The Top 100 version of The Final Battle! from Mario Party 4.
How to Unlock
Scan Koopa Kid's AR Icon from the M.U.G.E.N Trilogy website.