In Mercy, Rain (Wayward Children, #7.5)

by Seanan McGuire

Weakness is always easier to perceive from the outside, and every good predator knows how to pick out the most vulnerable members of a herd.

That was the night he’d stopped thinking of her as a duty and started thinking of her as a daughter. There’d never been a prayer for him, once he’d finally begun to see her clearly.

But a kiss from the dead means death to the living; as soon as their lips touched, her heart stopped.