Koopa Kid
The Koopa Kids (also known as Mini-Bowsers) are minions of Bowser who have only appeared in the Mario Party series. The Koopa Kids resemble miniature versions of Bowser (and are nearly identical to Bowser Jr. and Baby Bowser) and are usually attempting to crash the various parties Mario and his friends have. Originally the Koopa Kids were nothing more than minions indirectly fought by the Mario crew, but in Mario Party 5 one was finally made playable (despite this Bowser shows them no mercy when they land on his space).
Mario Party 5 also introduced Green K. Kid, Red K. Kid and Blue K. Kid who, as their name simply, are colored diferently. The aforementioned colored kids are one Koopa Kid split into three, its unknown if other Koopa Kids are capable of duplicating like this also.
It is unknown how many Koopa Kids there are, originally there seemed to be only a few, but Mario Party Advance seemed to imply that there are close to hundreds of them.
Koopa Kids are not to be confused with Bowser's children the Koopalings, Bowser Jr. or Baby Bowser who are all different characters.
