Olivia Olson

Olivia Rose Olson is an American actress and singersongwriter, known for her role as Joanna singing All I Want for Christmas Is You in Love Actually , and for her voice roles as Vanessa Doofenshmirtz in Phineas and Ferb and Marceline the Vampire Queen in Adventure Time. Olson is also a writer, contributing a section to the bestseller Adventure Time Encyclopaedia and an upcoming book for Cartoon Network.

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Source: Wikipedia