Kassius Konstantine is a human Imperial admiral from Coruscant, voiced by Dee Bradley Baker in Star Wars Rebels from the first-season episode "Gathering Forces" (2014). An Anaxes War College graduate and a founding member of the Imperial Navy's officer corps, he commands the Lothal sector fleet from the Star Destroyer Relentless.
Outmatched by a crew of six
His running failure to catch the Spectres is the show's standing joke about Imperial competence, but it is also doing structural work: it establishes that the Empire's problem is not resources but the incentive structure of its officer corps. Konstantine has everything he needs and no reason to take a risk, because a risk that fails is career-ending and a rebellion that grows is somebody else's problem next year.
Under Thrawn
When Grand Admiral Thrawn arrives, Konstantine's position becomes intolerable — outranked, tested, and openly found wanting after bungling the Mykapo operation against Iron Squadron. Jealousy does the rest.
Atollon
At the Battle of Atollon in 2 BBY he abandons his assigned position to chase the rebel flagship Phoenix Nest and claim the kill himself. Commander Sato rams the ship into Konstantine's Interdictor cruiser, killing him and opening the gap the surviving rebels escape through. He is the most literal illustration the show offers of Thrawn's thesis: the Empire loses to its own officers more often than to its enemies.