The dumbest part of this story is that there really isn't any good reason for him to have fallen in love with Yue. The story never gave a good reason for the relationship to be that way. Unless it's just suspension bridge crap and these characters are too stupid to know it. Which would make sense with the level of intelligence this writer must have to write this cliche trope filled story.
Ive been thinking the same thing since the beginning. the only other reason could be that hes sticking with his earth values of monogomy and chose her as his partner since she was the first to make her feelings know or simply to justify not starting a relationship with shea. but overall its just braindead story telling
Apparently the web novel has a ton of content about Yue and Hajime interacting (up to and including sexscenes) when they're still stuck in the Orcus labyrinth, yet virtually none of it made it into the manga.
The later half of the Orcus labyrinth arc feels like they were rushing things, anyway. Almost as if the series was close to getting the axe. Not sure if that was actually the case, but it sure feels like it. Especially the chapter where they meet Orcus himself; the entire climax happened off-screen, and the whole chapter consists of Hajime and Yue talking everything that happened. It basically screams "yeah, we got one chapter left to wrap everything up and make it look like an ending".
All that being said, you guys are absolutely right about how the manga handled the Yue/Hajime thing. By leaving out most of their personal interaction in the first arc, their relationship just comes out of nowhere. Hell, they don't even feel like a couple in the later arcs. Their relationship in the manga is just one big plot hole.
OTOH I kinda like that he's sincerely monogamous in spite of the cliché harem setup. Most other harem protagonists are either indecisive wimps, opportunistic perverts, or they just fuck every girl they can get their hands on. It's a change of pace, if nothing else.