Aberforth is Albus' and Ariana Dumbledore's brother, participant of the Battle of Hogwarts and owner of the "Hog's Head" Inn in Hogsmeade. He broke Albus' nose in a fight at their sister's funeral.
He sent Dobby to help Harry and his friends to escape the Malfoy Manor and helped Harry, Ron and Hermione get inside Hogwarts prior to the Battle of Hogwarts. He is the owner of Sirius' Mirror (bought from Mundungus Fletcher), the broken counterpart of which belongs to Harry.
His patronus is a goat.