One of the best things about Gilmore Girls is its characters, but there's one who was actually hated by fans of the show - April Nardini (Vanessa Maarano). Gilmore Girls started its run in 2000, telling the story of Lorelai (Lauren Graham) and Rory (Alexis Bledel), following the mother-daughter duo in the town of Stars Hollow, with all its weird and wonderful inhabitants along for the ride.
April was introduced in Gilmore Girls season 6, first appearing in the ninth episode of that year, "The Prodigal Daughter Returns". A precocious middle-school student, she waltzed into Luke's diner and wanted a sample of his hair for, shockingly, a DNA test, which later proved that Luke Danes (Scott Patterson) was her father. It was a major twist on what had been established about the character and Gilmore Girls in general, and April soon became the subject of fans' ire, although it's not entirely her own fault.
Part of the problem was that April was an annoying character, because, well, she was a kid with zero tact, which came off as irritating more than being cute or funny. April was supposed to be a mini-Rory, but without any of the attachment fans had come to develop for her; dropped into the series in season 6, she never really felt like a part of the Gilmore Girls world, like she'd wandered in from another show merely to disrupt things so late in the game. Which is, of course, another key reason April is so disliked: the break-up of Luke and Lorelai, and the start of Gilmore Girls' decline.
April's inclusion drove a wedge between Luke and Lorelai, thanks to his insistence on keeping his daughter separate from his fiancee. Although both tried to make it work, it led to the postponement of their wedding, a gradual distance forming, and a breakdown in communication, with April at the center of all of that. With Luke and Lorelai being the couple fans of Gilmore Girls had wanted to be together all along, and finally having their real shot at happiness together, it was April, more than either of the adults, who was blamed for the break-up, with even Morano saying she hated April for it. That in turn fed into many of the problems with Gilmore Girls season 7, and the overall decline of the series, with this as one of its worst ever storylines.
Not all of that is April's fault: as much blame lies with Luke, Lorelai, and April's mother, Anna, for how they behaved, when really April was just a child caught in the midst of it all who was getting a relationship with her dad for the first time. Combined with the fact that she was a badly-written character who got on almost everyone's nerves, and that this was a show that was designed to be familiar and comforting most of the time, it was a recipe that made April Nardini really hated by Gilmore Girls fans.