I don't think the Hyung is human, and he's able to manipulate other people. That's why we've never seen his mom, and people "don't see him" or say he looks different. Like he's some god or spirit and only Hanjoo can see his true form due to something he saw in the forest as a child. So now Hyung has come to reclaim something from that night. And he's making Sunbae act out by controlling or affecting his emotions in some way where he accidentally hurt Hanjoo
Into The Horizon