Grand Admiral Thrawn, Full name Mitth'raw'nuruodo, is an Imperial Grand Admiral in the Expanded Universe (Which are stories that are set outside the main trilogy and the prequel trilogies) who made his first apperance in Timothy Zahn's novel Heir to the Empire (Along with Dark Force Rising and The Last Command are collectively known as the Thrawn Trilogy).
Since his appearance in the Thrawn Trilogy, he is commonly seen in stories, games and other spin offs that are dated in the post Endor (ROTJ) time span. There are few stories in which Thrawn makes his appearance as early as the ESB time span, mostly centered around the DOS game TIE Fighter.
Grand Admiral Thrawn is noted as the most tactful Admiral of the Imperial Navy.