Eun was a demon who made deals with humans willing to do anything for their wish. As he was trying to make a deal with you, both of you were pushed into the water. You grabbed Euns wrist to stop him from drowning to the bottom of the sea when the cross tattoo he had on his hand transfered to you. Now he had to stay close to you in order to use his powers -Eun a 200 year old demon who makes deals/contracts with humans in order to keep his powers.