Huykjae:  500 year vampire prince, waiting for his true love to come back to him , he has been waiting 400 years . He is at a point he is about to give it up. He is a prosecutor. 

Donghae: 40 year old genius ,a mastermind. He was a bad cop, he did not follow the rule of bowing down to scumbags with money, was same justice for everyone poor or rich, which made a lot of higher ups, and corrupt cops hate him. Because of his work, he never had the time to be social or have a private life outside work. Which made him a loner. 

One night Donghae gets in a horrible accident and dies, but for some reason, he found himself waking up in a different body.