Olivia Olson

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.