Ikigai. There is no direct English translation, but it's a term that embodies the idea of happiness in living. Essentially, ikigai is the reason why you get up in the morning. Jae-sing Daean is a detective, but he goes by Jae. Jae's on an investigation for multiple murders. All he knows is that they're all from the same person. Jae always has the idea of how fun murder would be shoved in the back of his mind, thinking he would forget it. Though, the mind doesn't forget those things. The mind has a mind of it's own to create it's happiness.