Wendy O. Koopa

Wendy O. Koopa is presumably the third youngest of the seven Koopalings from the Mario games. She is the only female Koopaling.

Super Mario Bros. 3
Wendy is the third Koopaling to be fought. She is encountered on an airship in the water level. She fights differently than her brothers Larry and Morton, instead of shooting magic attacks, she creates energy rings that fly around the arena. Whenever she takes damage, she will create one more. She must be jumped on three times to defeat her. Once defeated, she runs off, leaving behind the wand she used to transform the king of Water Land.

Super Mario World
Wendy is the sixth Koopaling to be fought. Like Lemmy, she comes out of pipes to attack, with two decoys that look different in color from her. She must be jumped on three times to defeat her. Once defeated, she falls into the lava.

Mario & Luigi: Superstar Saga
Wendy somehow returns in Mario & Luigi: Superstar Saga, in Bowser's castle while Mario and Luigi are trying to get to Bowletta. Again, she is the sixth Koopaling to be fought. Once defeated, she apparently dies and vanishes like the others.

New Super Mario Bros. Wii
Once again, Wendy returned with the other Koopalings to kidnap Peach for Bowser. She is the fourth Koopaling to be fought, at a water level like in Super Mario Bros. 3. She fights in a similar style. She is fought twice. The second time, Kamek modifies the arena so that it occasionally fills up with water. Once defeated, Wendy fell over the edge. She survived, however, and was last seen after the credits alongside the other Koopalings, Bowser, and Bowser Jr., before Bowser's castle fell over and crushed them.

Physical appearance
Wendy has a red shell, and a bow on her head.

Trivia

 * Wendy's name is derived from Wendy O. Williams.