I hate that they call her a concubine, like she is his official wife (╯°Д °)╯╧╧

Maybe its just translation issues? Atleast I hope so

concubines can be wifes too? concubine doesn't necessarily mean that they aren't married (it depends on culture and stuff).
it is wrong however, in the sense that concubines are usually of lower status (like slaves etc) and "regular" wifes are free people. . .
but since they think she's a servant and not the daughter of the Saint, they are correct in calling her concubine