Elaborate
Basically popular theory was that Kiyoon/MC was the reincarnation of Muni. But Muni is a horrible character. It seems like what happened was they used a big snake/Imugi to try to revive Muni who died(young master/pretty ghost) ordered it, so they fed the sliced up corpse of Muni to the snake which combined the two characters(ultimately trying to revive Muni.) But I think it backfired cuz Muni soul got tainted. Hence that long necked ghosts evil appearence was probably a result of that(I don't think he reincarnated as the co-worker or at all) . Kiyoon, has red eyes, is shown to see the supernatural but rejects it and is shown to hate young master (Muni was obsessed with young master). I think Kiyoon memory of the past are fused with Muni's because he consumed him.
Mc is the snake