Olivia Olson (born May 21, 1992) is an American actress, singer-songwriter, and screenwriter, largely known for her voice roles as Vanessa Doofenshmirtz in Phineas and Ferb and Marceline the Vampire Queen in Adventure Time. She also played the character of Joanna in the 2003 film Love Actually and its 2017 short sequel Red Nose Day Actually. Along with this, she is also a writer, having written the Boom! Studios graphic novel Marcy and Simon, co-authored Adventure Time: The Enchiridion and Marcy…