Cliff Hooper was a crew member on the freighter Saul Tigh served on after the Cylon War.
According to Tigh, Hooper did not participate in the Cylon War due to the fact that his affluent father got Hooper a deferment, and because he didn't serve in the war Hooper was always trying to prove that he was a man.
This started a bar fight, which would have gone bad were it not for the introduction of William Adama, who intervened (Scattered, List of Deleted Scenes).