Lemmy Koopa

Lemmy Koopa is presumably the second oldest of the seven Koopalings, despite him possibly being the smallest.

Super Mario Bros. 3
Lemmy is the sixth boss fought in Super Mario Bros. 3. He flies an airship in an ice world, and transformed the king into a seal with a magic wand. Mario fights him and defeats him by jumping on him three times. Afterward, he runs off, leaving behind the magic wand.

This fighting style is similar to Wendy's, in that Lemmy sends attacks flying around the room, but this fight is much easier than Wendy because the floor is flat (since Lemmy walks on a ball) and his attacks (consisting of rubber balls) can be jumped on.

Super Mario World
Lemmy is the third boss fought in Super Mario World. He is fought at Vanilla Dome Castle. He fights inside pipes like Wendy does later on, with a fireball moving around. There are also two decoy Lemmys in the pipes, but these are scarecrows and are easy to distinguish from the real Lemmy. Once jumped on three times, Lemmy is defeated and falls into the lava.

Mario & Luigi: Superstar Saga
Members of the Koopa Troop never seem to stay dead; Lemmy and the other Koopalings returned in Mario & Luigi: Superstar Saga, not working for Bowser this time but Cackletta (who is possessing Bowser as Bowletta). Just like in Super Mario World, he is the third Koopaling to be fought by Mario and Luigi, and creates decoys of himself (except these are identical). Once defeated, he apparently dies and disintegrates like other defeated enemies.

Super Princess Peach
Lemmy and the other Koopalings were originally going to appear in Super Princess Peach (with sprites created for them already), but in the end they did not, with Super Mario Sunshine bosses like Petey Piranha, Gooper Blooper, and King Boo appearing instead.

New Super Mario Bros. Wii
Like his other appearances except his first, Lemmy is the third Koopaling to be fought, and like his first appearance, he is the boss of an ice world. During the battle inside his castle, Kamek magically enlarged Lemmy's walking ball, but Mario, Luigi, and the Toads were able to jump on him three times and defeat him, sending him falling over the edge. He survived, however, and is last seen in the ending sequence with Bowser and the other Koopalings (including Bowser Jr.) before Bowser's castle falls on top of them all.

Super Smash Bros 4
Lemmy Koopa has been confirmed to be an alternate costumes for Bowser Jr. in the video game; Super Smash Bros 4 for the WiiU and 3DS.

Trivia
In the Super Mario Bros. 3 manual, it is revealed that Lemmy would rather join the circus then destroy Mario, possibly making him the least evil Koopaling out of them all.

Name
Lemmy's name is derived from Lemmy Kilmister.