I just saw this in Lars’ route about Mark’s brother. At Lars’ dinner, Walter and his father mention Mark’s dead twin BUT, in Mark’s episode he keeps saying that his twin is locked in his room, his father says something similar (in one part he mentions how his twin asked to eat in his room like always). My question is: Why do Edgar and Lars say that Mark’s twin is dead, while Mark and Rai talk about how if he were alive AND WHY DID MARK HEAR SOMEONE’S VOICE IN HIS TWIN’S ROOM? I would say that maybe Mark somehow convinced himself that his twin is still alive and locked in his room because of the trauma of his brother’s death and Rai doesn’t know what to do so he makes Mark continue believing that, but as for the voice, I have no fucking idea! If anyone has any theories on this, let me know. I love hearing theories.
I think what you said could be what happened; it's just his coping mechanism (I remember he even mentioned going to a therapist and even took medicine, so yeah, it's that bad).
And about the voice, if I remember it correctly, he did start to accept the truth, and that voice is like a small intrusive side of him that wants to take revenge for what happened to Alex, but who knows, it could be one of those ominous characters we didn't meet yet.