(a.k.a. Yellow Koopa Troopa.)
AFFILIATIONS: Koopa Troop.
CLOSE RELATIVES: None.
SKILLS AND ATTACKS: Shell slide
STRENGTHS: In Super Mario World they were invulnerable to stomping while using their shell slide.
WEAKNESSES: They can still be swallowed and in Paper Mario 2, they could be knocked on their backs with a POW block or stomp attack.
Originally, Bowser hired them for their ability to become invincible to stomp attacks when they're inside their shells. The only problem was that they were still vulnerable to being swallowed by Yoshi. And, to make matters worse, they would give Yoshi all three different powers when eaten during their shell attack. Otherwise, they would just give him the stomping ability.
In Paper Mario 2, a group of K.P. Koopas appeared and were led by a friendly Koopa dude named King K(aka K.P. Pete). They weren't any different from regular Koopas and even lost that invulnerability they had during their last appearance. They could be a different species than the ones in Super Mario World or maybe just Koopas in yellow shells and shoes.