Halloween is one of the escaped teen-holidays from Holiday Island in "Depth Takes a Holiday." The character is an attractive, black-haired woman wearing an orange bustier and black skirt. In their band, The Holidays, she's the drummer.
She is one of the coolest kids at Holiday Island. She fled the island, bored to tears of it, but returned rather than let President's Day run the place. If she's not at Holiday Island, the real-world event of Halloween will quickly cease to exist (this didn't bother Daria much).
She's a surly 'bad kid' pastiche, and left to her own devices will organise raids with toilet paper & eggs on the houses of rich people. Creepy cobwebs appear in whatever house she's staying in.
In the Mexican dub, she was called Día de Muertos.