Se-In is a high-level demon that looks very human. He can hide his black wings. He loves Ryung and will do anything for him. When he was young Ryung Num found him beaten and helped him. He discovered Se-In was next in line to rule the demons. It is discovered that one demon is born with immense power each generation and eventually kills the current leader. The current leader tried to get rid of Se-In before he matured so he wouldn't be a threat. Se-In is severely injured when Dong-Young pierces him with her ice snow sword. He tried to kill Dong-Young for Ryung, but was killed by Doh-hyun and was turned into dust. Dong-Young seeing Ryung's true feelings realizes that he needs Se-In and regenerates him in a seed because he was given her blood she is able to. Ryung raised him as his son after that.