White, Alex (Novelist). Starmetal symphony #1

"Jazz pianist Gus Kitko expected to spend his final moments on Earth playing piano at the greatest goodbye party of all time, and maybe kissing rockstar Ardent Violet, before the last of humanity is wiped out forever by the Vanguards--ultra-powerful robots from the dark heart of space, hell-bent on destroying humanity for reasons none can divine. But when the Vanguards arrive, the unthinkable happens--the mecha that should be killing Gus instead saves him. Suddenly, Gus's swan song becomes humanity's encore, as he is chosen to join a small group of traitorous Vanguards and their pilots dedicated to saving humanity"--