Wendy O. Koopa, or simply just Wendy, is one of the Koopalings and the only female member of the Koopalings. She wears a pink polka-dotted bow on her head, large golden bracelets on her arms, a red pearl necklace and high-heeled shoes on her feet. Wendy has big pink lips too. She and Roy Koopa are the only Koopalings with pink shells, until in New Super Mario Bros. Wii, when Roy sported a purple shell instead.
She is thought to be a singer (as implied by the in-game text of Super Mario World, as well as the similarity between her name and that of punk singer Wendy O. Williams of The Plasmatics), and, in Hotel Mario, she is shown having a quick temper, and is so clingy with money and luxury that she set false coins (‘Wendy’s Pennies’) to stop Mario’s advance.
Wendy is one of the first Nintendo characters to distinguish herself as female by wearing a bow on her hairless head, and the first to do so in a game. Other characters who wear a bow include Birdo, who wears a similar bow, Lady Bow from Paper Mario, who wears a smaller bow on each side of her head, and Candy Kong, who wore a bow in Donkey Kong Country.
She is considered the first true villainess in a Mario game.
Super Mario Bros. 3
Wendy is the third boss of the game, after Morton Koopa Jr., where she hops about her cabin (in high heels) and throws candy rings (that aren't so sweet at all) at Mario, that bounce around the room. She has conquered Water Land (or Sea Side). To beat her, the player must still use the three jumps attack method, as usual. However, instead of magic rings she shoots red and white candy rings that bounce off the walls. Nevertheless, she will still be sent flying back home after three hits on the head.
Super Mario World
Wendy is the sixth boss of this game. She is the boss of Chocolate Island, where she attempts to confuse Mario by emerging from various pipes along with two decoys. Her fight is similar to Lemmy's except her pipes are all in a straight line and there are two fireballs bouncing around instead of one. She will still roast in the boiling lava under her after three bonks on the head. Her boss battle is thought to be easier than Lemmy's battle.
Wendy appears as the boss of Pipe Island. She is fought alongside a flying anvil. To hurt Wendy, you have to bean her three times with the anvil. The anvil must be overhead of Wendy to hurt her. Beat her and you get the Aquamarine Gem.
Mario & Luigi: Superstar Saga
Wendy is a boss after Bowletta takes over the Beanbean Kingdom. She has a time Bob-omb in her battle, along with Roy Koopa and Larry Koopa. she attacks with her rings that go under your feet and then close together, and with a duplicate program similar to Lemmy Koopa, but is made harder due to the time Bob-omb.
Wendy is the 3rd Koopaling to be recruited by Captain Goomba and the second-runner up of her Cutie Contest. Wendy is a ranged minion and is weak against melee minions but strong against flying minions.
Wendy appears along with the rest of the Koopalings in Mario & Luigi: Paper Jam where they assist Bowser and his paper self in kidnapping Princess Peach. Wendy is first seen after Mario, Luigi, and Paper Mario all reach Bowser's Castle but were knocked out by a giant cannonball. Wendy and Morton then have the heroes taken away to the Twinsy Tropics jail cell. Later, Wendy and Roy confront the heroes just before they left underground jail cell and battles them but loses.
Mario Kart series
Mario Kart 8
Wendy made her first appearance as a playable character in Mario Kart 8 along with the rest of the siblings.
Mario Kart 8 Deluxe
Wendy reappears in the second game. She is the only Koopaling to not have another Koopaling as her rival, but Peach and Daisy. She is controlled by the 200cc staff ghost Nin★Frank for Piranha Plant Slide.
In the cartoons, she was named Kootie Pie Koopa (a portmanteau of “Koopa” and “cutie pie”, also implying “cootie”), was apparently sixteen years old, and often whined about everything. Frequently, Koopa would execute his schemes (such as kidnapping Milli Vanilli) just to keep Kootie Pie pleased. She also displayed some sort of animosity or even envy towards Princess Toadstool (as Princess Peach was called in North American until 1995). There is little physical difference between Wendy and Kootie Pie. The only differences that really stand out are that Kootie Pie’s bow is more pointy and she has two fangs sticking out of her mouth. Kootie Pie also has a smaller bow on her tail.
Other than Lemmy, Wendy is the only Koopaling to appear in her own story in the comic books.
Wendy also played a prominent part in the Super Mario Adventures comic serial that ran in Nintendo Power in 1992. There, she continually matches wits with Princess Toadstool. Wendy is the de facto leader of the Koopalings in this comic, and stands out among her siblings, who are all portrayed with more or less identical personalities.
- Wendy is the 1-2 female Koopaling, the other being Pom Pom.
- Her feet are very small and mainly wears High-Heels.
- Although she has never seen without shoes, the other Koopalings, who are barefooted, confirm that Wendy has three toes on each foot, and also has white pads on the plant of her feet.
- Wendy was named after American musician, Wendy O. Williams. 
- Wendy is the only female Koopaling as well as Bowser's only daughter.