The Watcher is the guide and warden over War. The Charred Council not trusting War bound the Watcher to him. The Watcher is able to supress Wars powers and if needed kill him should he fall out of line.
He is depicted as being sadistic and wicked with no sense of mercy. The Watcher apparently had a sister who revealed to the Council War joining the fight on earth and the seven unbroken seals which resulted in her death by the Council for going to earth without permission which could explain the Watcher's harsh treatment to War and why enjoys having him as "a dog on a leash".