Jeannie II (her name in-series is merely Jeannie) is a recurring antagonist and nearly identical sister of the protagonist of the 1960's sitcom, I Dream Of Jeannie.
Some time after the relationship between Jeannie and Major Anthony Nelson has been established, Jeannie reunited with her sister, also named Jeannie, since, in this lore, all Genies are named Jeannie, or at least the female ones. At first delighted to have a member of her family back, Jeannie begins to remember that her sister was prone to antics both selfish and cruel. Both intrigued by Tony Nelson and generally wanting anything her sister had, Jeannie II was, unlike her sister, willing to put mortals in danger, if not outright mortal peril.
Over the course of the series, she was banished by various means, including recalling her own master, who she apparently could not directly disobey (though like her sister for Tony Nelson, she took a lot of liberties with the letter and spirit of his wishes). Since she was also played by Jeannie actress Barbara Eden, one of her favorite tactics was to change her normal appearance (brunette with a green Genie outfit) to that of her blonde, red-suited sister and trick Tony. For a time, she was able to fool Nelson, since his Jeannie's behavior was often mercurial, and her sister, who even to the end of the series was depicted as unworldly and naive.
While the two had wised up by the time of her final appearance, her schemes were in overdrive, and she was out for blood, having learned that the couple had finally become an actual couple and married. Bound again after these efforts failed, Jeannie II was not seen again in-series, though she tried once again during the first of two TV Reunion movies, I Dream Of Jeannie : 15 Years Later. In this case, the trap she set for her sister was undone by her nephew, Jeannie and Tony's son, who had enough Genie in him to release his mother from captivity. At first ignorant of his heritage, Anthony, Jr. had apparently known his aunt long enough that he did not think it odd that she had the same name as his mother.
As mentioned, Jeannie II dressed in green and gold, as opposed to Jeannie's iconic red-and-gold outfit, and had black hair to Jeannie's blonde. Eden, who reportedly loved playing this naughty twin, also spoke in a more seductive, arrogant voice in this role. Her character is similar to that of Samantha Stephens' cousin Serena (also played by Elizabeth Montgomery) from Bewitched, but much darker. While Serena was often more of a trickster than a menace, Jeannie II showed a willingness to kill (or place in direct mortal peril) her targets, even once causing her sister to spray Tony with a 'love potion' that actually made Jeannie hate him with a murderous vengeance, hoping to swoop in and save him while beating her sister once and for all.
In fact, Barbara Eden actually outdid Elizabeth Montgomery in one respect. While both played trickster twin cousins/sisters of their regular magical characters, Eden also played the two Jeannies' mother, who was presumably also named Jeannie.
Most antagonists on I Dream Of Jeannie were merely obstacles, or one-off foes capable of causing trouble but not mortal peril, or were misguided or zealous or both, like Nelson's NASA superior Doctor Bellows. Only Jeannie II and the Blue Djinn actually ever placed the heroes' lives in true, sustained, unequivocal danger.