Peter Pettigrew (a.k.a. Wormtail)
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Affiliation: Death Eater
At the end of Book 3, Wormtail has been unmasked as the person who sold out James and Lily Potter to Voldemort thirteen years before, and then faked his own murder to frame Sirius for the crime. Pettigrew/Wormtail transforms into a rat and manages to escape before anyone can take him to justice. But Wormtail is truly down on his luck. He has no options left and nowhere to turn. That's the only explanation for why Wormtail travels all the way to Albania to seek out the ghost of his old master, Voldemort. And Voldemort knows it, too, as he says to Wormtail: "Your devotion is nothing more than cowardice. You would not be here if you had anywhere else to go" (1.59).
Out of Options
Wormtail assists Voldemort every step of the way, even though the slimy, grubby Voldemort-thing "revolt[s]" (1.57) him. This tells us a lot about what kind of a person Wormtail is. He is profoundly weak. He is so desperate to be told what to do and to have someone looking out for him, that he'll willingly bathe and feed the monstrous thing that Voldemort has become for a whole year without any kind of immediate reward.
We can see proof of Wormtail's weak-mindedness in the way that Voldemort treats him. One day, Harry falls asleep in Divination class and sees a dream in which Voldemort berates Wormtail for allowing someone to escape (we later figure out that it must be poor Mr. Crouch, Sr.). Voldemort casts the Cruciatus Curse on his own follower – clear evidence of Voldemort's absolute power over pathetic Wormtail.
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After Wormtail has actually cut off his own hand for Voldemort, Voldemort still seizes the opportunity to remind him: "You returned to me, not out of loyalty, but out of fear of your friends" (33.44). It's only after letting him bleed for a bit that Voldemort finally gives Wormtail his new, strong silver hand. Wormtail whispers: "Master ... it is beautiful ... thank you ... thank you ..." (33.49). Wormtail is a truly damaged individual with essentially no sense of self. He's so eager to find security at the side of somebody with power that he's willing to maim himself to do it. Thus, we have some pity for him, even though he's evil enough to bring Voldemort back to life just to give himself a sense of power. Actually, we take it back – even if he's pitiful, we can't forget how much evil his weakness has brought about, especially for our dear Potter family.