So basically, he made a coping mechanism to her and she eventually projected the world in which the manga takes place, he finds out about it after the school is destroyed and, because of the coping mechanism, she thinks everything she is and their own relationship is broken down to pieces because they met because of the school and are there because of the school. He then is thrown at the club room and eventually fuses with the club room that became something like a heart (And he is inside it as shown in the last chapter), afer that he created his avatar just like Romi created her many avatars and the original Senpai is using the avatars to rewrite the world to one that is good enough for both of them to come back and exist, this is shown as one of her avatars that was born from a pettal instead of a clam. I think he is going to reconstruct the entire world with avatars of him and her instead of the sheeps and demon-clam-like things, because then they are not getting "Thrown at the masses" like she said but thrown at themselves, i think they are going to understand the world by living with only themselves, reflecting on themselves and then getting back to the real world as better human beings, capable of changing the "monsters" that assolate the real world or even better, her own coping mechanism world.