I haven't seen anyone point this out, so:
I don't remember if the manga already talked about it, but most of the cast member are supposed to be 16. And Rei used to be a salarywoman before coming to this world.
So wouldn't that mean someone who's in her early to mid-20s has a crazy romantic obsession with someone that's only 16?
I don't think this really needs the spoiler tags, but eh, just in case...
I mean, this is the case with most isekai (and other reincarnation fantasy), better just not to think too hard about this. It gets even worse when you consider those who gained their memories at a much younger age, some even at birth. In any case, they're old enough to know what they're doing both in their world and in most saner parts of ours.