Here's my crazy crack theory: ml (white hair) was probably some aristocratic kid and mc was his servant, yooyoung (dark hair) or his family took over politically or forcefully and mc sent ml away for his safety, then gave himself over to yooyoung, who maybe loved him at some point but was ultimately abusive. Ml comes back and tries to get together with mc, but something happens that leads to yooyoung's death, perhaps caused by ml with a curse or a spell, and mc takes the blame for it and bears the karmic debt into his next life (alternatively ml could've cast a spell to "ensure they'd end up together", and that'd be the curse that brings folly to anyone close to mc besides ml himself, mc dies shorty after yooyoung and carries the curse into his next life. Mc could've killed himself before he can fulfill the purpose of the curse which is to be together with ml, which left ml stuck with unfinished business and lingering as a ghost). Present yooyoung might've actually been trying smth with mc and if succeeded he might've ended up abusive like his past self, but regardless he got into the accident, died, or at least his soul was weakened enough to leave his body vacant, then ml's soul that hadn't reincarnated took his corpse and pretended to be him, but since I assume the reason he lingered was due to unfinished business, he's wacky and weird and has some memories of yooyoung's body but hasn't fully assimilated, his soul is also only focused on his business (being with mc forever) so when they're not together he starts to fall apart
Here's my crazy crack theory: ml (white hair) was probably some aristocratic kid and mc was his servant, yooyoung (dark hair) or his family took over politically or forcefully and mc sent ml away for his safety, then gave himself over to yooyoung, who maybe loved him at some point but was ultimately abusive. Ml comes back and tries to get together with mc, but something happens that leads to yooyoung's death, perhaps caused by ml with a curse or a spell, and mc takes the blame for it and bears the karmic debt into his next life (alternatively ml could've cast a spell to "ensure they'd end up together", and that'd be the curse that brings folly to anyone close to mc besides ml himself, mc dies shorty after yooyoung and carries the curse into his next life. Mc could've killed himself before he can fulfill the purpose of the curse which is to be together with ml, which left ml stuck with unfinished business and lingering as a ghost). Present yooyoung might've actually been trying smth with mc and if succeeded he might've ended up abusive like his past self, but regardless he got into the accident, died, or at least his soul was weakened enough to leave his body vacant, then ml's soul that hadn't reincarnated took his corpse and pretended to be him, but since I assume the reason he lingered was due to unfinished business, he's wacky and weird and has some memories of yooyoung's body but hasn't fully assimilated, his soul is also only focused on his business (being with mc forever) so when they're not together he starts to fall apart