Sinestro is a former member of the Green Lantern Corps who was banished for his abusive use of power and subsequently became a villain. Before his fall from grace, Sinestro was once one of the most respected members of the Green Lantern Corps. He was assigned to mentor Hal Jordan, the Earth-based Green Lantern, and the two became close friends. However, Sinestro became increasingly authoritarian in his approach to enforcing the law, using fear and intimidation to control those under his watch. When his behavior was discovered by the Guardians of the Universe, the leaders of the Green Lantern Corps, he was banished to the anti-matter universe.
In the episode "In Brightest Day," Sinestro returns to seek revenge on the Green Lantern Corps and his former protégé, Hal Jordan. He allies himself with the villainous Doctor Polaris and attempts to conquer the Earth using his yellow power ring, which is fueled by fear. Sinestro is eventually defeated by Hal Jordan and the Justice League, and he is banished to the anti-matter universe once again. Throughout his appearance in the series, Sinestro is depicted as a cunning and manipulative villain, willing to do whatever it takes to achieve his goals. He is shown to be highly skilled in using his power ring, which allows him to create energy constructs and manipulate matter. Despite his villainous actions, Sinestro is portrayed as a complex character, with moments of vulnerability and regret over his fall from grace.