Groot is the Iron Giant
Marceline, from Adventure Time is the little girl from Love Actually
Ferb is the little boy from Love Actually AND he's Jojen Reed
Eliza Thornberry is Gretchen Wieners
Katara, from Avatar: The Last Airbender, is one of the 7 Evil Exes Scott Pilgrim has to fight. You might also know her as Anne Veal, but probably not.
Prince Zuko is Rufio