A dictator of Earth's history, later than Hitler, whom both Spock and Garth of Izar list among the great conquerors in the same breath as Alexander, Caesar and Napoleon.
Lee Kuan was a tyrant who tried to seize absolute power at some point between the 20th and 23rd centuries. Nothing is said about where or when he ruled, but his name had become a byword for conquest by Kirk's time.
He is mentioned twice in the Original Series. In Patterns of Force, Spock tells Kirk that the urge to dominate recurs throughout human history and counts off Ramses, Alexander, Caesar, Napoleon, Hitler and Lee Kuan. In Whom Gods Destroy the insane Garth of Izar boasts that he will surpass them all and gives much the same roll, adding the name Krotus at the end.
Because the lists run in chronological order, Lee Kuan is usually placed in the turbulent 21st century. He is never seen.