The Traitor's Tail
Peter Pettigrew was described as being as plump, fumbling fellow, not very good at magic and often compared to Neville Longbottom. However, with the help of his friends, he did manage to master the animagus spell. With it, he could shapechange into a rat and with this small form, was able to deactivate the Whomping Willow and allow his friends to pass on their nightly escapades.
He was entrusted with being James and Lily Potter's Secret-keeper, a trust he betrayed by turning them over to the dark lord Voldemort. After this betrayal, it was believed that he had, in fact, been spying for You-Know-Who for some time before that. He was not suspected because of his pathetic grasp of magic and pitiful demeanor.
After faking his own death and framing Sirius for it, Peter hid himself in his rat form, somehow becoming the familiar of Percy Weasley, who passed him down to his younger brother, Ron. Named Scabbers, he lived as Ron's rather dull familiar for years until he was recognized by Hermione's familiar, a large and very intelligent cat named Crookshanks.
Since it worked so well for him before, Peter faked his own death again, leaving blood and cat hair all over Ron's bed to frame Crookshanks. He was later discovered by Hermione though, and from there fell into the hands of his one time friends, Sirius Black and Remus Lupin.
After some very close calls with Sirius, Peter managed to escape again and returned to his master. He was last seen, helping in the resurrection of Voldemort after the capture of Harry Potter. Despite hie reputation as a coward, he still had the stomach to complete the awful spell, including cutting off his own hand. Last reports say that his master has healed him, and like his long-tailed new kin, there's no telling where he'll appear next.