Nobody

Fourteen-year-old Arianna Price, rescued, bloody and terrified, from a bathtub by a stranger after witnessing the brutal slaying of her father and brother during a home invasion, had no time to ask questions of the man who'd interceded on her behalf before disappearing, without a trace, into the night. Twenty-five years later, her future and profession dictated by that single event, Special Agent Arianna Price of the FBI is determined to bring the still unidentified vigilante, her erstwhile savior, in for questioning.
Except he seems to have never existed. He has no name. No past. No identity. He appears during horrific acts of violence, intervenes, then vanishes like a wraith. Evidence suggests he's been doing so for thirty years, or more. Yet, according to witnesses, he has never aged. He is punishment. He is vengeance. He is Nobody.