He wears a skull mask to hide the scars on his face he recieved by originally trying to fight his fate as a slave. His collar is used to keep him in check by Maleria and his staff was a gift from The Old Wizard. The gem on his mask is the source of his magical powers.
The Gate Keeper has been a slave to Maleria for thousands of years and does so to keep his family safe. He has asked Maleria for only one favour: that his children are not raised in slavery. She did grant this but at a price, she did not allow him to ever touch them, instead allowing his offspring to be sold at Blackheart Hollow to be cared for by Subetans.