Wormtail (Peter Pettigrew)
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We think one of the worst things about the Order photograph that Moody shows Harry is that Wormtail is in it, surrounded by people who trusted him, including Harry's own parents. Of course, we find out in Book 3 that it was Wormtail who sold out James and Lily Potter to Voldemort. Wormtail also framed Sirius for his own murder and then went underground for twelve years in the shape of a pet rat ("Scabbers"). Wormtail has now rejoined Voldemort and made it possible for Voldemort to rise again. So he's really gone over to the Dark side, it would seem.
In Book 5, we mostly see Wormtail in the Pensieve flashback to Professor Snape's worst memory. In it, Wormtail appears as an awkward boy flattering his more powerful friends. None of them – neither James, nor Sirius, nor Remus – seems to take Wormtail seriously at all. The fact that his friends don't take him seriously is why Wormtail is able to do so much damage later in life: none of them can believe that he would be brave or capable enough to join Voldemort and work against his own friends.