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Dawn, Buffy, the Vampire Slayer
If you thought Nikki and Paolo's introduction on Lost was too abrupt, imagine tuning into the fifth season of Buffy and discovering the Slayer had a sister you never knew about! In all fairness, even Buffy didn't know about Dawn since she hadn't technically existed before. Viewers had little sympathy for Dawn's teen angst and soon grew tired of her. And frankly, the Scooby Gang had enough on its plate without having to live through a Judy Blume novel.