Ngao

Wasawat is the devil because in his previous life he committed equal amount of good and sin. His job is to punish people for their sins and he basically sets up bad people in real life to give them a second chance. He wants to move on, but in order to do that he has to find someone to replace him. Someone that is gray, not too good (white) or not too bad (black) they have to be in the middle and possess both good and bad qualities.
That is where he tries to get Isara to replace him because he is like Wasawat, equally bad and good, but because of Charoenkwan, the woman Isara loves, Wasawat in the end gave up on the goal. Wasawat had a past with everyone including Chalinee, Isara's half sister.
(Source: asianfuse.net)