Harry Potter and the Half-Blood Prince
Affiliation: Order of the Phoenix
Lupin is a werewolf working as a kind of undercover cop in the werewolf community. You see, the werewolves are, for the most part, on Voldemort's side, and they are headed by a vicious leader, Fenrir Greyback. Greyback bit Lupin after Lupin's father offended the leader, thus making Lupin a werewolf for life. Once upon a time, Lupin had sympathy for Greyback, believing he could not control himself, but now he knows that Greyback is intentionally vicious. Lupin hopes to collect the information that will prevent Greyback from causing any more harm.
Lupin is very serious and rational. When Harry relates the story of what he saw happen between Snape and Malfoy, Lupin says, "It isn't our business to know […]. It's Dumbledore's business. Dumbledore trusts Severus, and that ought to be good enough for all of us" (16.69).
Tonks, an Auror and a fellow member of the Order of the Phoenix, is in love with Lupin. However, it appears that he's hesitant to start a relationship with her because he's a werewolf. Following Dumbledore's death, Tonks publicly declares that she doesn't care that he's a werewolf. Though he's a little gruff, Lupin has a big heart and is fighting for good.