In the season 2, we saw when Zoom captured Wally west and threatened to kill him if Flash doesn’t give up his speed force, he complied and then Zoom/Hunter Zolomon became the fastest man alive across the multi-verse. He tried to kill Barry Allen but Caitlin intervened and he took her to Earth-2. Killer Frost (Caitlin Snow Doppler-ganger) was also locked in Zoom’s cage along with the mysterious man in the iron mask, she cut a deal with Caitlin to help her break free from the cage in return for a safe return to Earth-1. When she came out, she turned on Caitlin and tried to kill her only to be stabbed by Zoom.
The executive producer Andrew Kriesbergwas asked about the possible character arc for Caitlin Snow, he hinted at the possibility of Caitlin’s mom and her doppler-ganger coming into the show. Both Caitlin Snow and her Earth-2 doppler-ganger share estranged “Icy” mothers.
Kevin Smith, a popular director who directed an episode of the Flash, is shooting another episode for the upcoming season. He tweeted an image which revealed the title of the episode and it’s Killer Frost. This triggered a speculation that Earth-2 doppler-ganger of Caitlin Snow/Danielle Panabaker would return to the show. But how can she come back ever if she is dead? The fans speculated that she can return as a result of Flashpoint, maybe she is another Killer Frost in a multi-verse (maybe from Earth-3).
Well, it’s clear now how Killer Frost is coming back. In the last episode titled “Paradox”, we saw “the Rival” going after the Flash after he lost his speed powers in the alternate timeline. In the post-flashpoint timeline, he is traveling like a regular guy but having visions of a previous life of a speedster. He meets Dr. Alchemy who gave his speed powers back using the philosopher’s stone and activated him to defeat the Flash, but once again he failed. In the episode, we also saw Caitlin Snow hiding the fact that she is turning into a meta-human in the present timeline, she was an Ophthalmologist in the Flashpoint, it’s likely that her powers have been activated by Alchemy as she was also struck by particle accelerator explosion like Cisco.