
Jaheon was the one responsible for taking care of the pot mugo was trapped in. During those times, they formed a sort of friendship shared between two people in misery (parallel to the current one). The "sin" that the main branch talks about, and why jaheon's branch family was treated cruelly and sacrificed, is instigating mugo to take revenge (ch. 9). But the other mugos don't remember this, only the 4th one.

The way the kang family named the mugos intrigues me. They call glasses mugo 1st mugo and big dick mugo 2nd mugo, but from what big dick mugo said, he was awoken by gamer mugo, or "the monster," screaming.
"The monster was petrified so i put him to sleep," he said.
So my theory is that the kang family or whoever wrote those books didn't actually know which mugo came first. They just know which mugo appeared before them. That's why 1st mugo is glasses mugo because he's the most amiable so he's the most likely one to sit down and hold a conversation. They don't know about 4th mugo because he never appeared in front of them.
Oh i totally knew what character he was going for when he said "elite delinquent" lolololol that's scarier than regular delinquent bc regular delinquent just beats you up but elite delinquent can get away with it AND be your boss someday
Yeah it’s like they type even if you have evidence against them, nothing happens to them ( ̄∇ ̄")