Danielle Panabaker’s Killer Frost on The Flash gave the Arrowverse its best redemption story. Killer Frost started out as a villain but has since evolved into a capable member of Team Flash - and is now just as important to the team's survival as Caitlin Snow is. Like the Elongated Man (Hartley Sawyer), Killer Frost is one of the superpowered heroes that Barry Allen (Grant Gustin) takes with him into the field. Her abilities have made her a valuable asset against foes that even Flash has trouble defeating.
Killer Frost hasn’t always had the relationship that she currently has with Barry, Cisco (Carlos Valdez), and the others. Killer Frost was originally introduced as Caitlin Snow’s Earth-2 evil doppelganger. Through Flashpoint in The Flash season 3, Barry created a timeline where the primary version of Caitlin shares her body with a new Killer Frost persona. Whenever Caitlin would become the DC Comics villain, Team Flash was forced to deal with her. For a while, she struggled with the fact that she had to coexist with Killer Frost, and eventually looked for a way to “cure” herself. However, over time she changed her mind and developed an understanding with her dark side.
Through Killer Frost, The Flash delivered a compelling redemptive arc. For most of season 3, Frost was depicted as a ruthless metahuman who appeared to be devoid of any emotional attachments. She quickly became an enemy of Team Flash and tried on more than one occasion to kill them. She also allied with season 3’s main villain, Savitar, to defeat Barry. Despite her cold and heartless countenance, she had second thoughts about Savitar’s intention to kill Iris (Candice Patton) and ultimately turned against him, thus beginning her road to redemption.
Following Savitar’s defeat, Killer Frost changed considerably in The Flash season 4. As Savitar pointed out, she never actually killed anyone during her time as a villain, so she was never too far from being a good person. Even so, becoming a hero didn’t come easy for Frost. She resisted Team Flash’s attempts to bring her into their family but eventually came around. She came to understand that not only could Barry, Cisco, and Iris accept her as own separate person, but that Caitlin could accept her as well. By forming relationships with the various members of Team Flash, Killer Frost was able to feel like she really was a part of their family and that she was much more than just Caitlin’s metahuman alter ego. Also, her friendship with Ralph Dibny in seasons 5 and 6 furthered her character development and helped her discover herself.
The Flash was able to craft a story that properly redeemed Frost. In fact, her redemptive story was handled better than other Arrowverse characters who had to go down the same path. The Arrowverse tried and failed to redeem Arrow’s Malcolm Merlyn (John Barrowman) and the repeated twists and turns in Black Siren’s arc ruined her redemption story. As for Killer Frost, her evolution from a villain to hero felt right for the character, as she was never inherently evil to begin with. Much of what made her bad stemmed from her feelings about being an unwelcome side of Caitlin Snow. What Frost wanted was her own identity.