Thrawn was a Chiss, a species of aliens from the Unknown Regions. He was a brilliant military strategist and commanded the remnants of the Imperial Fleet in a series of stunning victories against the New Republic. His study of his enemies' artwork gave him insight into their thought processes and cultures, allowing him to create highly effective tailor-made strategies.
Grand Admiral Thrawn is described as a tall, solidly built humanoid with blue skin and glowing red eyes. Because of the Empire's prejudice against non-humans, he was the only non-human to reach the Imperial Navy's highest rank – that of Grand Admiral. Not surprisingly, Thrawn's promotion was long kept secret by Emperor Palpatine, and as late as five years after the Battle of Endor the New Republic appeared to be entirely unaware of his identity, or even his existence. He was also the last Grand Admiral promoted by the Emperor; he earned the rank because of his instrumental role in stopping Grand Admiral Zaarin's attempted coup, receiving the late Zaarin's rank and position as a reward. As a Grand Admiral, he wore the white uniform befitting his rank. This is distinguished from the standard charcoal grey or olive colored uniforms that Imperial officers wore.
When Thrawn choose the Chimaera to be his flagship. He immediately left his mark on the ship, having a luxury entertainment suite once used by Pellaeon's predecessor turned into a meditation chamber/command room for his personal use. Thrawn's reorganization of the fleet placed the Chimaera at the center of his armada, along with the Star Destroyers Death's Head, Judicator, Inexorable, Stormhawk, and Nemesis. Thrawn soon began training the crew to his own specifications. Afterward, he began his tremendous campaign on the New Republic. The Chimaera remained his flagship throughout the devastating campaign.