In the fictional universe of The Foundation Series, Cleon I (11,988 GE - 12,038 GE) was the last Galactic Emperor of the Entun dynasty. He was Emperor of the Galactic Empire when Hari Seldon first arrived on Trantor. He succeeded to the Imperial throne in 12,010 GE at the age of twenty-two following the death of his father, Stanel VI, who was fortunate enough to escape the roughly one-in-two chances of assassination faced by the last century of Galactic Emperors.
Isaac Asimov portrays Cleon as an amiable man, no longer enthusiastic about the trappings of office, eager to treat others as his equals and yet not capable of conversing comfortably—or even understanding their motivations. In his genial but misguided well-wishing.
Arguably, the most significant event of Cleon's rule occurred in 12,020 GE, when the young mathematician Hari Seldon came to Trantor and revealed that Psychohistory was a theoretical possibility. Cleon I’s reign represented a curious interval of quiet between troubled times. This is undoubtedly due to the skills of his Chief of Staff, Eto Demerzel. Cleon would never have learned of this development by his own efforts; insulated from the Galaxy and confined to the Imperial Palace's grounds, he relied upon his able Chief of Staff, Eto Demerzel, to conduct the Machiavellian maneuvers required to keep the Galactic Empire functioning. Demerzel became aware that Seldon could possibly develop a mathematical method for predicting future history, and he (indirectly) informed the Emperor, who took an immediate interest in the matter. While Cleon was content to use Seldon as a source of self-fulfilling prophecies—a tool to prevent unrest—Demerzel had come to believe that the Empire itself was dying, and that only an honest investigation could prevent its fall or at least minimize that fall's effects. Therefore, Demerzel took steps to promote Seldon's development of his infant science.
After Demerzel's retirement, Cleon appointed Seldon to be his new Chief of Staff, a position now termed First Minister. (Prelude to Foundation states that Demerzel had no formal title, while its sequel, Forward the Foundation, makes him the First Minister—a minor inconsistency as far as the Asimov canon goes.) Seldon exerted himself to the fullest, employing pragmatic measures to halt anti-Imperial conspiracies while simultaneously developing psychohistory into a system capable of making solid predictions.
Unfortunately, Cleon's personal ineptitude became his downfall. Wishing to thank a gardener, Mandell Gruber, for his part in thwarting an assassination attempt on Dr. Seldon, he promoted Gruber to Chief Gardener over the Imperial Palace grounds. Gruber, not wishing to abandon his lifestyle, overcame his timidity and shot the Emperor. The chaotic upset which followed Cleon's assassination saw the rise of a Military Junta and Hari Seldon's retirement from overt politics.
(12,010 GE - 12,038 GE)