Iggy Koopa

Iggy Koopa is presumably the fourth youngest of the seven Koopalings.

Super Mario Bros. 3
Iggy is the fourth Koopaling to be fought. He is encountered on an airship in Giant Land. His battle style is similar to Larry's. Once defeated, Iggy ran off, leaving behind the magic wand he used to transform the king of Giant Land.

Super Mario World
Iggy is the first Koopaling to be fought. To defeat him, Mario knocked Iggy into the lava.

Mario & Luigi: Superstar Saga
Iggy somehow returned in Mario & Luigi: Superstar Saga, and appears in Bowser's castle when it is under Bowletta's control. Again, he is the first Koopaling to be fought. Once defeated, he apparently dies and vanishes.

New Super Mario Bros. Wii
Once again, Iggy returned alongside the other Koopalings, kidnapping Peach for Bowser. He is the fifth Koopaling to be fought, in a forest level. He is fought twice. The second time, he is riding a cart being pulled by a Chomp, which Kamek uses a spell to increase his size. Once defeated, Iggy fell off the screen. He survived, though, and was last seen after the credits alongside Bowser Jr., Bowser, and the other Koopalings, before Bowser's castle fell on top of them and crushed them.

Physical appearance
Iggy wears glasses and has rainbow hair. He initially has a blue shell, but in later games, his shell and hair are changed to green.

Trivia

 * Iggy's name is derived from Iggy Pop.