Blackstar by Joshua Viola

In a world blasted and barren, the last bastion of civilization is a fortress-city called Central. Kaine, the city's benevolent arbiter, rules over the people in exchange for his gift to them: Re:memory—a public archive containing humanity's memories of the world before it was destroyed. Rezin doesn't know who he is or why he's in Central, but he does know this: he is a Reaper. When Rezin meets Elara, an intrepid adventurer with a troubled past, and Vray and Bastian, mysterious twins possessing incredible powers, they embark on a journey through space and time in search of the answers Rezin seeks.