10. Kara Zor-El / Kara Kent
It's ridiculous that we even had to consider putting a character like Supergirl on this list, but as much as I hate to say it, fans didn't take too fondly to Kara when she showed up in season 7. Laura Vandervoort did a fantastic job bringing an incredibly comic-accurate version of Kara Zor-El to the small screen, capturing her confident, punch-first-think-later attitude really well. It highlighted how different she was to her cousin Kal-El, too.
However, not every Smallville fan was familiar with the source material, so when Kara arrived and completely upended the established dynamics, many of them weren't thrilled - especially as she often clashed with Clark and questioned his methods. She also got between the Chloe Sullivan and Jimmy Olsen relationship, which created even more conflict.
The good news is that she did find a place for herself on the show before the end of the season, winning over viewers and becoming a great hero in the process, so when she eventually returned (as an early version of Supergirl) in season 10, fans were happy to see her.
It really should be said that Kara was designed to be a divisive personality early on and Vandervoort understood the assignment well. We're just glad she got the love and appreciation she deserves for it.