This character is a mistress of another character.
A mistress is a relatively long-term female lover and companion who is not married to her partner, especially when her partner is married. Generally, the relationship is stable and at least semi-permanent, but the couple does not live together openly and the relationship is usually, but not always, secret. There is often also the implication (if not the fact) that the mistress is "kept" – i.e. that her lover is paying for some (and sometimes all) of her living expenses.
This role does not preclude a character from also being a i371, i1493 or a courtesan.