Darr by Theresa Beachman

He’s humanity’s best hope. He just doesn’t know it yet.

Violet was one of the British army’s top snipers when the insectoid Chittrix invaded Earth. She almost died in the first wave.
Traumatized, she takes refuge with a small group of survivors in an underground bunker near London, struggling to find a reason to live beyond waging guerilla warfare against the hated insects. Vengeance is not enough to make up for her losses, but it’s all she has.
Darr, always a lone wolf even before the invasion, lives with a secret that threatens to destroy him both mentally and physically. For the safety of everyone around him, he leaves the survivor’s community and avoids all human contact.
Living with the threat of death hanging over his head takes its toll. But going back is not an option.
When a Chittrix ambush traps Darr in close quarters with Violet, they band together to survive. With his abilities and her accuracy, they make a formidable team as bullets and sparks fly.

But the secret between them is a constant threat and soon, it will explode.