Hey hey hey, let's get some of the facts right here, okay. People get stabbed in the front most of the time, when they screw around with harems. It's only backstabbing if there's plausible deniability, that's totally out the window, when the perpetrator smiles at you saying "Yeah, I did that."
That was Saioji's backstory, found in the woods, chased off a cliff. There's a history of people just popping over from Japan, and there's lots of reincarnators around. It's not unusual for them to be spreading their culture around.