Affiliation: Order of the Phoenix
Albus Dumbledore's odd and prickly brother, Aberforth, turns out not to be quite as strange as he's rumored to be (though we haven't gotten to the bottom of his whole goat obsession, nor do we want to). He's a good guy at heart, but his tormented family life and the disappointment of Voldemort's resurgence have combined to make him something of a cynic. Though Aberforth is clearly sympathetic to Harry under his gruff exterior, and his cooperation makes the whole rebellion possible, he's still resistant to the idea that his brother was doing the best thing, for Harry or for the world at large. And Aberforth's got good reason to doubt Albus, too – we learn the truth about the Dumbledore family from him, and it seems as though their fraught history has given Aberforth good cause to wonder what his brilliant brother was up to.
In the end, though, Aberforth's true heart gets the better of him, and he joins in the fight against Voldemort – his protestations couldn't stand up to the real injustice of Voldemort's regime, and he has to give in and assume his old role as an Order rebel. His assistance throughout the novel and his participation in the events of the Hogwarts battle prove that Aberforth is just as good-hearted, determined, and brave as his brother, even if he's a little wackier.