Se-In

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 on 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 severally injured when Dong-young pierces him with her ice snow sword.[Source]