Mandy (originally from The Grim Adventures of Billy and Mandy) is a devious, maniacal little girl, who once tried taking over the KND in The Grim Adventures of the KND crossover, turning it into the MND (Mandy New Dictator). She fused herself with the Delightful Reaper monster and became all-powerful, but she was quickly defeated by Nigel and Grim, but she got away when they tried to arrest her.
She used a giant robot designed like herself to destroy Grim and Numbuh One in The Grim Adventures of the KND.
In Trickquestion's universe, Mandy was the main antagonist of Operation: JUSTICE, leading armies of child villains to destroy KND and take over the world. She stole Grim's scythe and brainwashed Kuki to use it to be Master of Death, so that Mandy and her henchmen would be immortal. Mandy became an all-powerful demonic being, but her power was lost when the villains rebelled against her. Nigel defeated her by trapping her inside her own sphere that can only be opened from the outside by her voice. The sphere protects whatever's inside from anything, even death, meaning she's trapped in there forever.
In Gamewizard's universe, her full name is Mandy McKenzie, and she is Rachel McKenzie's cousin. Mandy's father is Rachel's paternal uncle. As revealed by Darkrai, Mandy is a fearbender. Rachel was always disappointed that she couldn't convince Mandy to become good, but Mandy hardly cared. After the events of the crossover, Mandy led her Boogey Pirates (which she became captain of since Big Boogey Adventure) across the seven seas, wreaking havoc. In Operation: DUTCHMAN, she made the group fight in her arena in the Pirate Fleet, and offered the Manaphy Egg and a Talisman as the prize. She later tried to steal Davy Jones' Heart to stab it.
She was one of many in Truth or Dare: KND Plus Others.
She gave her ship, the Boogey Bay, wheels so that it could race in The Great Galactic Race.
In the one-shot "Mandy's Nightmare", she had a nightmare where she ruled the world, and Rachel and the KND operatives sang a song about her "glory". Darkrai also revealed she was a fearbender.
In Legend of the Eight Firstborn, Mandy learned fearbending in the Dream Realm from The Chronicler. she found out about Lord Gnaa's troops attacking the world. She and her pirates sailed through the Kremling fleet to get to Cleveland, Virginia, where she demanded Katie take her to Nightmare Land. Katie brought her to Wesley Dodds, who was making a ship to travel to Nightmare Land, and they went with him. When they reached Nightmare Land, they made their way into the Fear Factory, where they defeated the Crackjack monster. They soon met up with Darkrai, who revealed that Mandy was a descendant of Princess Zelda, and that she holds the Triforce of Wisdom. During the Freedom Saga, Ghirahim found Mandy and brought her to Ganondorf, convincing her to come with the promise of Rachel's destruction. Clockwork later gathered Mandy up with the rest of the main benders as he brought them to the Isle of Elements, where they met The Chronicler. During Mandy's test on the island, she decided to give Rachel a second chance, and she returned to Rachel to make amends. However, she was later kidnapped by Ganondorf as they used her in the ritual to absorb her soul and awaken Demise's powers within Ganondorf. When Rachel and Lehcar came to save her at Castle Hell, Ganon possessed Mandy's body and used it to fight them. Ganon was defeated and Mandy's soul was saved. When Ganondorf became Satandorf, Mandy was empowered with the Triforce along with Rachel and Lehcar, and they vanquished Satandorf together.
During the 20-year timeskip, Mandy has become a Logia fearbender, and a feared Pirate Emperor of the "Boogey Sea" at just 13 years old. She battled Marine the Raccoon while she captained the Raccoon Pirates, aiming to take the Licorice Stalks' Moon Seed. Katie was also a member of her crew. During the fight, Mandy, Marine, Katie, and Henrietta figured they were missing something, so Mandy abandoned her plan and returned home to Billy. She and Billy eventually married and had a daughter, Cindy Cortix.
In The Great Candied Adventure, Mandy appeared at Moonrise Platform just after Sheila Frantic defeated her daughter. Mandy used her Logia power to put the Big Mom Pirates in a nightmare before clashing with Henrietta.
Mandy is a powerful Logia fearbender who has control over nightmares, able to turn her own nightmares in her favor, and give other people nightmares. Her favorite attack is the Scare Stare, able to terrify anyone to her will and put them in nightmares through just a simple exchange of glances. Mandy also wields a pirate sword and is quite proficient with it. Mandy is also holder of the Triforce of Wisdom, and can unleash great powers of light if she joins forces with Rachel and Lehcar.