He was always seen at the bar of Hyperions cantina and apparently made a good Mai Tai (according to Graven Hill and Bralik). Free Bird is one of his favorite jukebox tracks and he's puzzled by the notion of people using zerg bones as trophies.
Cooper was a member of Raynor's Raiders since at least 2500. He saw Jim Raynor with Zeratul once, the Dark Templar making him feel uneasy. He was also surprised when he learned that Raynor used to be a Mar Saran marshal.
During the Second Great War, Cooper had concerns that Raynor was drinking too much, but he would later abandon them, even offering him a rare Umojan beer. The presence of Gabriel Tosh was an unwelcome one.
At some point before the Flashpoint Conflict, Cooper betrayed the Raiders to the Dominion for money, serving as a spy aboard the ship.
They were repeatedly engaged by the rest of the Dominion Fleet, led by Arcturus Mengsk himself. Raynor decided to take shelter at Deadman's Rock, where Matt Horner's "wife", Mira Han, ruled. Cooper learned their destination from Valerian. Only three rebels, plus Kerrigan, actually went to the planet.
Cooper contacted Gary Crane, Han's traitorous lieutenant, and convinced him to try to capture or kill the rebels. The plot failed, and despite Cooper's assistance Crane's rebellion was defeated.
The rebels then headed to the Moebius Foundation Space Station Prometheus. Arcturus Mengsk learned of their location and attacked them there. During the battle, Cooper volunteered to fly Wraiths against the Dominion. This allowed him to get off Hyperion and defect. The Raiders discovered his treachery when he broke formation and refused to communicate; Horner allowed the bartender to escape to the Dominion flagship, White Star.