2. Chillblaine
If you’re sensing a pattern here, congratulations you figured it out: A lot of the worst heroes in the Arrowverse did indeed originate in the final few seasons of The Flash.
It’s not that they couldn’t have been great characters, it’s just that they weren’t explored with any real nuance, which makes it hard to buy them as characters much less superheroes. Chillblaine is the perfect example of that, as he turned up as little more than a criminal who tricked Frost. Yet the show pursued a romance between the two.
He’s been fine when used sporadically, but his promotion to a regular character in season 9 was one of the season’s many missed opportunities. Not only was the “being a superhero” narrative forced upon him as well, he repeatedly betrayed Team Flash. Mark never showed any desire to be a superhero, and even though the death of Frost was a strong driving force for his change of heart, the storyline let him down.
He was also written out of the season for a significant amount of time, having a real portion of growth take place off-screen. And then he returned and watched Team Flash defeat the bad guys in the series finale. He didn’t mount any offense or defense whatsoever.
The show never once justified the decision to make him a regular character and unfortunately set him back from the growth he had achieved in previous seasons. Chillblaine didn’t need to be a superhero; he would have been fine as a shady ally that Team Flash could occasionally rely on.