Mandy is one of the two titular characters from The Grim Adventures of Billy & Mandy.
The Grim Adventures of Billy & Mandy
S1E12a "Big Trouble in Billy's Basement" (this is also the first time Mandy is barefoot). After Grim gives her a manicure, she puts her foot on the table for a pedicure.
S2E5a "Grim in Love"
In "Chickenball Z" Mandy eats a magical chicken-ball that gives her super powers for karate. She tears through the rest of her competition.
S3E2b "No Body Loves Grim"
S3E5a "Nursery Crimes"
S3E5b "That's my Mummy"
In "Billy Ocean" Mandy wears sandals on the beach.
"Prank Call of Cthulu"(Note: The reason why Mandy is barefoot is because Grim said that while people get suck into the phone, their shoes never go with them because shoes are too big to enter.)
In "My Fair Mandy", Mandy enters a beauty contest. Swimsuit portion!
In "Spidermandy" Mandy is barefoot in her pajamas. As you could guess by the title, Mandy turns into a spider.