It depends on the culture. For most, concubines were publicly known and were considered honorable positions, as they helped the ruler/noble produce plenty of heirs. The male heirs with the wife/queen/lady still took priority position, otherwise it was the eldest male with any concubine. Concubines were eventually considered prostitutes in more recent times, and their use in royal/noble houses fell out of favor.I mean, I'm not an expert, but from what I understand, a concubine is someone you're in a relationship with, regardless of whether you already have an official partner. I'm just not sure whether that kind of relationship is supposed to be open or kept hidden.
Anyway, even if the word fits the situation, I was actually joking. you know, just swapping one weird word for another. But thanks for taking it seriously!
A lot of times, the more powerful or wealthy you were, the more concubines you could afford to take care of (yes, they did get taken care of. Very well, actually). A lower lord might only have one, for example. To my knowledge, someone couldn't take a concubine until they were married. Otherwise, she's just a prostitute to have fun with. So yeah, concubines were literally just for breeding.
So for the sake of this game, the correct translation is fuck buddy (or whatever people call it these days) unless Inoda wants to get married and keep Asuna around just to breed her.
