Olivia Olson (born Olivia Rose Olson; May 21, 1992) is an American actress and singer-songwriter known for her role as Joanna singing "All I Want for Christmas Is You" in Love Actually (2003) and for her voice roles as Vanessa Doofenshmirtz in Phineas and Ferb and Marceline the Vampire Queen in Adventure Time. Olivia is also a writer, contributing a section to the best-seller Adventure Time Encyclopaedia and an upcoming book for Cartoon Network soon to be announced.