Who is the werewolf in Harry Potter?

Remus Lupin

Remus Lupin is the famous werewolf character in Harry Potter. He is the Defense against the Dark Arts teacher in "Harry Potter and the Prisoner of Azkaban".

He has no control over whether or not he turns into a werewolf. When there is a full moon, he turns.

He is not an Animagus. They are people who have the magical ability to turn to animal form at will. His good friends chose to learn to be Animagi as a form of moral support for him.