Morton Koopa Jr.

Morton Koopa Jr. is presumably the second youngest of the seven Koopalings, the seven kids of Bowser who appear in several Mario games.

Super Mario Bros. 3
Morton is the second Koopaling to be fought. He is encountered in an airship at Desert Land. His fighting style is similar to that of his younger brother Larry. Once defeated, he runs off, leaving behind the wand he used to transform the king of Desert Land.

Super Mario World
Again, Morton is the second Koopaling to be fought. He is fought in a room where the walls are slowly closing in, and often comes onto the ceiling. To defeat him, as in Super Mario Bros. 3, Mario must jump on him three times. Once defeated, he spiraled into the distance and exploded in a cloud of smoke.

Mario & Luigi: Superstar Saga
Morton returned in Mario & Luigi: Superstar Saga, working for Bowletta in Bowser's castle. As usual, he is the second Koopaling to be fought. Once defeated, he apparently dies like the others, and vanishes.

New Super Mario Bros. Wii
Morton reappeared in New Super Mario Bros. Wii. This time, he is the sixth Koopaling to be fought, and is not the boss of a desert land, but a mountain land. Like the others, he is fought twice per level. The second time he is fought, Kamek casts a spell on the floor so that whenever Morton pounds the ground, the two parts of the ground on either side of him will go up and touch the ceiling, crushing anyone who is standing on it at the time. Once defeated, Morton fell off the screen, but survived, as he was seen alongside Bowser, Bowser Jr., and the other Koopalings, before Bowser's castle fell on top of them and crushed them.

Physical appearance
Morton usually has a grey shell and three curls of hair on his head. He also has brown skin and a white head.

Trivia

 * Morton's name is derived from Morton Downey Jr.