Elias is a vampire, but not your average run of the mill vampire. He and the rest of the vampires in The Fold were created for the sole purpose of serving as guardians of witches. Elias is a Nightwarden. He's been awakened from his stasis to serve as Nightwarden to a witch he can barely stand. But that doesn't stop him from his duty.
Until she goes missing. Failure is not an option.
Mariya's sister is High Sorceress of their coven. This doesn't frustrate Mariya. What frustrates Mariya is that she's inexplicably drawn to the strong, silent type—specifically, her sister's Nightwarden, Elias.
Nightwardens do not engage in relationships. Witches do not consort with Nightwardens.
When Mariya's High Sorceress sister is kidnapped, Mariya is forced to work in close proximity with Elias to get her sister back forces feelings to rise to the surface.
Feelings that are dangerous, because a Nightwarden cannot fail, and he cannot engage in relationships. The penalty is permanent and irreversible. It's a penalty Mariya doesn't want him to have to pay that penalty. Meanwhile, she has a sister that needs saving.