Broken Angel (House Phoenix) by S.W. Vaughn

Gabriel Morgan’s sister, Lillith, has been missing for two years-and he’s been searching for her every minute of them. So far, he knows only that she is somewhere in New York City, she is a prostitute, and she is growing close to a dangerous criminal organization known only by a symbol…a five-colored star. But when he finally finds the organization, he is taken hostage along with his sister. Forced to participate in a series of brutal street fights in order to win her freedom-and his-Gabriel must reprogram his body and his mind to become the ultimate fighting machine.