I'm not on Marcus' side. The person he loved is long dead and gone... It's not fair to her current self nor to the current hero. She isn't the same person that the demon king fell in love with and he needs to move on. He doesn't even know the current version of "Jun", as the Jun of the past and the current one are two completely different people, with different sets of memeries, only sharing memories though reincarnation, and different personalities because of their different pasts...
So no, I don't want Marcus and current "Jun" to end up together. He didn't do anything wrong, so I want him to have a satisfying ending in which he is defeated and laid to rest.
It's like the game SOMA- the character might have shared memories up until a certain point but are different people beyond that. It's a really good game, check it out!