Demon Familiar

Supernatural Bonds #7:

Pulled from the void by a binding spell, demon essence becomes mortal-familiar, and Ianthe forms, yielding to her preference for the female shape. She longs to live a human life, to love as a human, and while she's escaped the dark realm and the lord she called master, she can't escape her nature.

Incubus, succubus, Ianthe is able to shift between male and female forms, though both aspects need to be fed. She could have any man, but it's Homicide Detective Miguel Torres...