In Loving Absolution. Cryptid Keeper Series, #5

Callum Campbell knows exactly what the monster wants. He feels it constantly, coiled beneath his skin, tasting the air, craving. The Slaugh does not merely share his body. It shares his mind, his senses, his every waking moment. And when Ellie Braden walks into a room, the hunger that rolls through him is not his own. It is the beast's. And it wants her dead. For years, Callum has kept himself at the edges of the world; not only to protect others, but to punish himself for what lives inside him.
The isolation is penance. The self-loathing is familiar. What he was not prepared for is Ellie refusing to stay at a distance, bright and stubborn and wholly unaware that every time she looks at him, the Slaugh sharpens like a blade. The creature has chosen her. Not because she is weak but because her soul is luminous, and the Slaugh wants to extinguish it. Now Callum is out of room to run. He can sacrifice himself and gamble that his death takes the beast with it.
He can disappear again, deeper this time, far enough that Ellie can never follow. Or he can do the thing that terrifies him most: surrender to a past he has spent years hiding from, and trust that facing what he's done is the only kind of man worth loving. There is no clean choice. There is only which part of himself he is willing to lose and whether love, stubborn and scarred and still standing, is reason enough to try.