Great series.
Separate bedrooms though?
(apparently that's also more of a japan thing)
That'd be pretty common anywhere, I'd think. They weren't formally dating when they moved in, and even if they were it's still not unusual to do in your early 20s. They probably just switch between one bed or another.
The part I don't buy is that they'd be able to afford this setup in Tokyo on their salaries while raising a kid and an occasional freeloader.