My understanding is that Lee Muhan is the original body - 1 human soul, 1 human body.
But then the people tried to make a God using the Mugo Curse which is broken into three parts: Torture, Isolation, and Veneration
They chose Lee Muhan as the victim and started to torture him but he couldn’t handle all the pain so his mind fractured into who we know as Monster Mugo (Born from the Torture.)
Lord Mugo (or Lust Mugo) was born from the Isolation and Darkness because Monster Mugo couldn’t handle the darkness of being trapped. (Born from Isolation.)
And finally, Sir Mugo (or Godly Mugo) was born from the veneration and praise after the isolation. It’s why he’s the most proud to be a god. (Born from the Veneration.)
Think of it like a mirror being hit with a hammer:
Monster Mugo was the first crack in Lee Muhan mind, then Lord Mugo, and finally Sir Mugo!
Lee Muhan’s soul was so close to finally being eradicated for good but he’s clinging to the very little memories he has (most of which I believe are of Ja-Heon) to stay attached to this body, this broken mirror.
Hope that helped!(๑•ㅂ•)و✧
TL;DR — It’s 1 body all around, but 1 soul broken into three with the last small piece of the complete soul clinging to small memories like a parasite instead of vanishing.
So when Mugo was a human he was called Lee Muhan, afterwards while being tortured the 3 sir Mugo personalities appeared as a way to protect himself. From my understanding the fourth personality Lee Muhan isn’t a personality, he’s what’s left of the Lee Muhan’s soul and memory that’s survived to try and keep them functioning. Idk tho

I'm starting to get confused, does Mugo originally have 3 entities in 1 body or what is 4 entities in 1 body? It's hard to understand what the Green hair guy is saying, because I'm so confused