Gollum

Gollum is a nasty little creature, a thief and a murderer. Born as a hobbit named Sméagol (Also referred to as Slinker, Stinker, the Sneak) he lived with other hobbits by the river Anduin. One day his best friend and cousin Deagol found a ring at the bottom of the river. Sméagol killed him and took the ring. The ring became "his precious" and he fled with it into the caves of Moria. After living in total darkness for nearly half a century and influenced by the ring (which had been made by Sauron) he transformed from Sméagol to Gollum. No longer a hobbit, he stayed alive in the mountains feeding on fish and goblins until the day he met Bilbo Baggins... His part in the adventures of the Fellowship is ambiguous. He was, of course, pursuing his own goals, but one can not always accuse him of treachery. Frodo's mission would have never been completed without Gollum who bit off Frodo's finger with the Ring on it and fell into the Cracks of Doom.