Unless I'm wrong, The MC would be "wolfie", and the ML would be the brown hair guy with the sister.
If I'm not mistaken, there are 2 scenes you may be refering to:
• 1: when the manga showed someone dead in the MC's arms, or "wolfie" as you called him. (MC's mother standing over them saying this is what happens when you get involved with humans)
• 2: this new scene of the MC in a cave with someone.
To my understanding, those scenes would have taken place like hundreds of years ago back in the Joseon Dynasty. So maybe an ancestor of the ML
I think people say Mc for the ml cuz for the vast majority of bl the Mc is the bottom so yeah, justice for our true Mc
The girl from the wolfie's past is not our MC but his sister. And MC is a man who met our ML.