Russian Spy Gilligan (Gilligan Vs. Gilligan)
A Cold War-era Eastern Bloc spy made up to look just like Gilligan, he at first assumed that emulating the dim-witted first mate would be as simple as saying whatever foolish thing came into his head. This proved not to be the case, since no one else is quite Gilligan in the way that Gilligan is. The spy's masters were working under the mistaken assumption that the castaways must have some hidden agenda in being there - said benefit of the doubt being rather foolish in this case. The spy did not get along with his direct superior, and was almost left behind when his mission was suddenly scrapped, his awkward undignified flight as his boat sailed finally making him at last truly like Gilligan.
Fake Thurston Howell (Will The Real Mr. Howell Please Stand Up?)
As the castaways listened to yet another plot-driving radio broadcast from back home, they were all shocked to hear an interview with their own Mr. Howell, none more than Howell himself, who was right there with them. A masterful con artist claimed to have survived the shipwreck of the SS Minnow, and now widowed, sought to live the high life with Howell's money. The real Howell was apoplectic, even attempting to swim back to the mainland in his insanity. As fate (and plot device) would have it, the hard-partying fake Howell fell off his yacht, ending up on the island, lucking out once again by finding and taking the real Howell prisoner. When the real Howell escaped, the classic 'No He's The Phony' situation was resolved when the impostor panicked after a news report exposing him came over the radio. While he escaped with some of Howell's money and credit cards, he was unable to spend them. While managing to get back to the mainland, he was apprehended for fraud and sentenced to twenty years. It was also implied that he was very much the worse for wear, the prospect of him being able to impersonate Howell again made slim by his rough journey. At episode's end, both Gilligan and Lovey Howell were still concerned that the real Howell was the impostor, albeit in a comical way in keeping with the show.
Eva Grubb/Fake Ginger Grant (All About Eva)
A introverted girl named Eva Grubb made her way to the island, thinking to escape the scorn of being labeled plain-looking and unattractive. The castaways were immediately interested in how she got there, but she did not wish to leave the isolation she had sought. Efforts to trick her into letting them leave while sending someone back for her later failed (thanks to Gilligan), so they plotted instead to make her wish to leave by giving her a makeover. Ginger, Mary-Ann and Mrs. Howell turned all their efforts to this goal, succeeding beyond their wildest dreams when Eva remade now looked just like Ginger. However, with her new beauty came arrogance and avarice, and a desire to take over Ginger's life. Imprisoning Ginger (it seemed all three impostors needed their targets alive for a time), she worked on convincing everyone she was who she wanted to be, but lack of practice and her poor eyesight without glasses (and she had no contact lenses) made the others suspicious, and of course, the real Ginger returned. Exposed, Eva offered up the most sincere apology and tears she could fake, and everyone was fooled. Leaving the island, the episode ended with it seeming that Eva had returned to the mainland and succeeded in taking Ginger's place in Hollywood, leaving Ginger despondent. That said, when Ginger and the others returned home in 1979, her career, while stalled, was still an option for her, so it seems likely that Eva was again exposed at some point.
Sort-Of False Professor (A Very Brady Sequel)
In the feature film A Very Brady Sequel, the plot centers on an impostor who claims to be Roy Martin, Carol Brady's first husband, a claim the gullible (The two Brady Bunch films are affectionate parodies of their source material) family accepts easily. As it turns out, he was a betraying friend of Martin's who ditched him on a storm-tossed ship that is almost stated outright to be the SS Minnow from Gilligan's Island, making the impostor the almost-Eighth castaway. As if to cement this, Carol refers to her college instructor husband as the Professor, and a wealthy man played by John Hillerman says his son, lost in the same shipwreck, was Gilligan. However, none of these statements fit with the details of the characters' lives as stated on Gilligan's Island. While no details of Gilligan's family (including whether Gilligan was his given or family name) were ever revealed on-screen, the Professor's name on a few occasions was Roy Hinckley. The non-canonical status of these events, already in a parody, is further brought home at the end of the movie when a woman claiming to be Mike Brady's first wife shows up - and is played by I Dream Of Jeannie's Barbara Eden, as Jeannie in all but name. On the other hand, since all three shows were produced by Sherwood Schwartz, these connections cannot be entirely ruled out either. Besides Jeannie's blonde hair contrasting with the dark hair of her three supposed sons, none of them showed magic powers, although genetics (Jeannie's own sister had dark hair) and Genie lore about hybrid children allow for some wiggle room. However, pictures of the dead spouses seen in early Brady Bunch episodes do not resemble either Russell Johnson or Barbara Eden.
All three original castaway impostors have something that unites both them and the other visitors to Gilligan's Island : Not one of them aided the castaways in actually getting off the island, with their various betrayals and mishaps perhaps exceeding Gilligan's stupidity as a cause for their 15-year tour.