Koopa (also known as Koopa-Troopa) is the main and only playable character of the game, A Koopa's Revenge, but later joins forces with Goomba and Shy-Guy in the sequel, A Koopa's Revenge II. His leader is Bowser , which is who he searches for in the first game. Koopa despises Mario and his gang for unexpectedly crashing Goomba's birthday party.
In A Koopa's Revenge
Koopa is a yellow turtle that usually wears a green shell although he has some variations. When Koopa uses a power-up, his shell changes color. His shell becomes Orange (AKR2) and Red (AKR1) when equipped with Fire Flower, and when equipped with a Feather, his shell becomes Purple (AKR2) and Blue (AKR1) and grows wings. Without a shell, Koopa becomes a beach koopa which is seen wearing light blue boxers.
A Koopa's Revenge II
Koopa is still the same, although his shell colors are mostly different. Abilities are more in-line with the Mario franchise. For example, Fire Flowers now make Koopa shoot fireballs and Feathers still give him wings, but now acts more like the cape/tanooki from Super Mario Bros. 3 and Super Mario World respectively.
A Koopa's Revenge 3
Nothing is much known but Koopa will breathe fire again like in the first game. Others will be seen later