A "one time" Green Lantern, Sinestro was the best of all Green Lanterns, that is until he became power hungry. Sinestro was stripped from his ring, and he eventually found the yellow element, which gave him more power.
Thaal Sinestro is a fictional character from the DC Comics Universe. He was created by Gil Kane and John Boome, making his first appearance in Green Lantern #7, in early July of 1961. A Green Lantern and former mentor of Hal Jordan, Sinestro was one of the most powerful Green Lanterns in the universe until he was de-powered by the Guardians and later became the arch-nemesis of the entire Green Lantern Corp
Thaal Sinestro, originally an anthropologist specializing in reconstructions of ruins of long-dead civilizations from the planet Korugar , had a natural affinity for order as displayed by his meticulous reconstructions of ancient ruins. Sinestro, like his counterpoint Hal Jordan, became a green Lantern after his wounded predecessor, Gosgotha, crash-landed and gave his ring to Sinestro, who managed to defeat the Lantern's pursuer. When Gosgotha requested his ring back after the battle in order to keep him alive, Sinestro refused and instead allowed him to die and in doing so, took his place. The Guardians of the Universe, unaware of his actions, allowed Sinestro to become the Green Lantern for sector 1417.