The Flash: 10 most disliked storylines, ranked
3. Caitlin's "death" and Khione's arrival
I don't know what's worse: The fact that Caitlin Snow's death was so obviously a fakeout or the fact that they wanted us to believe it was real when they behaved so disrespectfully towards her? Both are correct and both are awful.
The Flash's final season made many missteps but perhaps the one that received the loudest response from fans was the fact that it seemingly, quietly, uncaringly killed off Caitlin and then reintroduced Danielle Panabaker as a pointless third character named Khione. It was a ridiculous creative choice given the limited time to complete the show's overall story and it was incredibly disrespectful to Caitlin to sideline her in the final season (and then not even bother to have her friends and family grieve her apparent death).
Khione herself wasn't an offensive character but she never justified her existence. The fans just couldn't wrap their heads around the senselessness in the decision-making here, because why would you waste your final season on integrating a new character who ultimately left at the end of it anyway? Honestly, points were made by those fans.
Khione was both overpowered and powerless at the same time, incapable of helping in the battle and also ending the battle with a click of her fingers; it was all very contradictory, with most of her growth taking place off-screen. Danielle Panabaker was charming as the character but that couldn't even justify this pointless storyline. Like most of the plots in season 9, it took away from the show's legacy instead of adding to it.