Firmus Piett is a human Imperial naval officer aboard the Super Star Destroyer Executor, played by Kenneth Colley in The Empire Strikes Back (1980) and Return of the Jedi (1983). He begins as a captain, is promoted to admiral in the middle of a sentence when Darth Vader executes Admiral Ozzel, and dies at Endor when a Rebel fighter goes through the Executor's bridge.
He is a small part with an outsized reputation, largely because the two films use him to show what serving under Vader actually costs: Piett's competence is real, and it buys him nothing except the chance to be next.
Catalogue entry. This figure has a verified record here rather than a full write-up — the encyclopedia labels its own depth instead of padding it.