Honestly, past muni having a twin or brother could be possible too. And it seems like the evil spirit who claimed that he's the "real" muni, didn't tell the whole truth to giyun either. Cuz their wordings were kinda manipulative.
My theory is that evil spirit is actually a snake spirit(??), who might be bound to muni's soul because of the ritual. And maybe cuz muni couldn't move on from young master, the snake also had to become wandered spirit for centuries.
However, the soul in giyun is trying to protect him by making him forget/prevent him from remembering how he truly died. Probably if giyun remembers, the curse(??) might resolved and the snake spirit could be free then prey on others. But this is still a theory though (￣▽￣")
Oooo this ones good!!
At some point I can put things together but why is he getting called brother now? All I can think of now is that Giyun was originally muni and had a twin, who views himself as muni. Or, not
Cause it wouldn't make sense if his grudge is wanting the young master love him when clearly the current lifetime is definitely attracted to giyun. Then technically speaking he already is receiving the love no? Somebody tell me how they are perceiving this story at the moment