Merc by Elaine Levine

Merc is back in Valle de Lágrimas, the Valley of Tears—the Colombian hell hole he knows all too well. Having survived years of rebel wars and illegal cartel activity, it faces a new and worse threat, an enemy long known to the Legion: the Omni World Order. Merc’s job—and that of all Legionnaires—is to protect humans from Omni mutants, not each other. But Merc doesn’t do rules, and the chaos he unleashes takes on a life of its own, making it impossible to throttle down.

Ashlyn DeWinter is excited about her trip to Colombia. Valle de Lágrimas keeps popping up in her research, but her friends warn her it isn’t safe for tourists. Something about the village draws her nonetheless. Her visit comes in the wake of an unusual man’s time there—a man who did terrible and wonderful things to protect the village. A man whose energy she hungers for, though she hasn’t met him and likely never will. A man like that couldn’t possibly exist—could he?

A better question is—how does she find him when the only name she has for him is his odd nickname: Merc?