The Tournament - The Interstellar Trials, #1

Her life is a crime. To survive, she'll steal another.  Reggie's spent her life stowed away in the crawl spaces of a generation ship that's headed to the stars. If the ship's tyrannical authorities discover she exists, it'll mean a long walk out of a short airlock.  When the authorities announce a tournament to make peace with the estranged other half of their fleet, Reggie steals another girl's identity -- along with her spot in the games.  She doesn't want to win; she wants to escape to the other fleet.
But avoiding the trials is harder than she expects, and avoiding the other contestants is even harder -- especially the charmingly naive future captain who keeps trying to bring her into the fold.  When a threatening note appears under her door Reggie must decide how ruthlessly she's willing to be -- and who she's willing to betray to protect her secret.