Isekai's the wrong place to expect deep characters and proper world-building. Like watching a B-horror movie that cost 100€ to make and expecting quality on par with Alien.
"I have dedicated my entire life to tracking down and killing this specific demon"
*kills demon in 2 chapters*
"Joke's on you, there's more of me around, and they have even less personality and development."
This reminds me of a similar oneshot, except it had some drama because of the foreign girl's parents being against their relationship, with the father slapping the daughter in front of the japanese girl and her parents.