Princess Mandie (pronounced "Man-die") is an evil alien princess that Mark Chang left on the altar because he considers her beautiful appearance to be hideous. It is later revealed in "King Chang", she only wants to marry Mark so that she can cast him aside and rule Yugopotamia herself.
Mandie is the princess of an alien planet of Boudacia (a pun on bodacious) and is what is to be considered as a "Deadly Beauty". She somewhat resembles a science-fiction variation of the TV character “Xena: Warrior Princess”. She carries a flaming sword which is her preferred choice of weapon, and yet she possesses many other kinds of weapons such as bazooka guns, a double-bladed axe, a bomb, and she even grows diamond claws on her fingernails, similar to the Marvel Comics characters "Sabretooth" or “Lady Deathstrike” (or the villain Nadira from Power Rangers Time Force). She also has rockets built into the boots that allow her to fly.
Princess Mandie stands about the height of a human adult. She has yellow skin, elf-like ears, and a pair of curled antenna on her head resembling that of a butterfly. She wears an emerald-colored corset, skirt, garter on the right leg, and knee-high boots. She also has crystal earrings and a crystal headdress, and she wears black lipstick. She has long purple hair and green colored pupils. She is a very beautiful girl.
She was looking forward to marrying Mark Chang of Yugopotamia. However, once Mark fled the planet and left her at the altar, her true colors came out. When her heart gets broken, she becomes extremely angry and vengeful that she’ll kill anyone in her path, and what she hates the most is being left alone at the altar. She would find that person who she is after and either forces them to marry her or kill them, which is why her name is pronounced "Man-Die" instead of the normal name "Mandy". Despite her rather beautiful appearance, she has an extremely aggressive nature.
In "New Squid In Town", Mandie (then pronounced Mandy) was engaged to marry Mark on Yugopotamia, yet her husband-to-be fled the wedding and left to Earth leaving Mandie alone at the altar. This is because although she is considered beautiful on earth and elsewhere she is thought to be hideous to Yugopotamians. Angered and heartbroken, she swore that she'll find Mark and force him to marry her or he’ll die. After this point, she is referred to as Man-die. She first found Mark on earth in the Dimmsdale Elementary school cafeteria where Mark was not in his disguised form, however, Timmy wished everyone in the room would freeze. Shortly after that, Timmy and his fairies take Mark somewhere safe and back in disguise while also presumably sending Mandie somewhere else for a temporary amount of time. Later on, she found Mark, Timmy and Cosmo and Wanda at the Dimmsdale Dump and after scanning Cosmo and Wanda’s power levels, she dealt with them first by freezing them in carbonite. Timmy and Mark tried to get away from her in his spaceship but she was catching up with them quickly, but Timmy was able to trick her by tricking Crocker to wear the disguising belt which disguised him as Mark which fooled Mandie by taking the disguised Crocker in Mark's place back to Yugopotamia. During the wedding, however, when Mandie kissed Crocker on the cheek, the disguise on Crocker wore off, furious for being tricked, she sent Crocker flying back to earth landing in Principal Waxleplax’s bedroom.
In "Five Days of F.L.A.R.G.", she didn't come back to Earth because she didn't know Mark was on there thanks to Mark's disguise. But she was part of the Yugopotamian’s celebration of F.L.A.R.G. which was the reason why Mark couldn't go back to celebrate it with his people because of her.
In "What's the Difference?", when Mark’s disguising belt was broken and no longer in his human disguise, Mandie was able to track him down to the school library where he was with Timmy, Cosmo and Wanda. In a desperate situation, Timmy wished the entire school were like the puzzle for three-year-olds. However, the wish also hid Cosmo and Wanda’s wands, so they were on a race against time to find the wands and get things back to normal before Mandie finds Mark and forces him to marry her, and as well prevent Crocker from finding the proof of Fairy God Parents at the same time. Both ended up in front of Crocker’s classroom where they found Mark disguised a trophy and Mandie was planning to blow up Dimmsdale with a bomb. Timmy tricked Crocker to get the Mark “trophy” back from Mandie while he and his Godparents looked for the wands in the classroom. Once they found them, they wished everything was back to normal and that Mandie was sent back to her home planet and have no idea where Mark was, but...
In "King Chang", Mark's father is trying to be assassinated so he comes to his son to help take his place in the throne while he is away hiding and Timmy and his fairies investigate the situation. It is immediately revealed (and pretty obvious) that it was Princess Mandie that was responsible for the attempted assassination. She then shrinks the fairies rendering their powers useless and forces Mark to marry her which is finally successful. Immediately afterwards, her true motives (debatable to be true or not among fans) were to take away the throne, naming the planet as "Mandiepotamia" and to use it to conquer the galaxy. She then locked Timmy and Mark away in the dungeon, Mark, believing that there is no one strong enough to take on someone like her, is reassured by Timmy that “there is another”. After Timmy and Mark escape and rescue the fairies, they flee into a ship back to Earth with Mandie hot on their trail. Mark gets into his disguise and is caught by Mandie, but before she could blast him, Vicky shows up telling Mandie that Justin (Mark’s new form name) is her man and makes an example by literally beating Mandie up. Mandie defeated, arrested and taken into custody for her treachery and everything is restored back to normal. As well, Mark can continue to live on Earth and not be exiled as well.