Maybe he's a friend-gone yakuza who decided to take his old crush by force now that he's lost all his fucks to give. I mean, he's already committing crimes as seen in the part where the profiler old man calls him and asks for his assistance in apprehending a person and i think the scar on his face is supposed to clue us in on him being yakuza. If this IS the case i could see it going two ways, either it's going to end up (probably FAR into the story) being a cute/creepy story about a yakuza taking his wife and said "wife" learning to accept and be okay with the situation or its going to maintain the suppressing ambience of the story so far and the poor guy's just going to be imprisoned until he loses his mind.
Holy shit this is intense. I'm really hoping that it ends up a little more fluffy, but the intensity sure got my attention from the start. I'm curious as to whether or not he's purposefully trying to induce stockholm syndrome on him. I think he was a classmate that was one of his "inner circle" back in high school that the MC forgot about over time as shown in the last few pages of ch. 4, my reasoning for this is the captor's reaction to the MC not remembering him, he gets PISSED. It's possible he was a bully to the MC back in high school and THAT is why he's pissed that the MC doesn't remember him, but i don't see that as the likely scenario as in the flashbacks so far the MC seems to be pretty popular, because of this i think it's a friend that he forgot.