Kang Yise's Happy Ending

In a world of angels and demons, Kang Yise was granted immortality and has lived for centuries. He now runs a detective agency under a pseudonym and helps people find information. One day, a fallen angel named Kwak Rowan comes to him as a client. And much to Yise's surprise, Rowan asks him to find the whereabouts of Kang Yise so he can eliminate the latter! Despite not being able to die, Yise is still baffled by Rowan's desire to kill him. Can he hide his identity long enough to find out why?