Caitlin is fully suited up as Killer Frost on The Flash, with the white hair and blue lips that makes other super villains jealous. In "I Know Who You Are," Caitlin's alliance with Savitar brings her closer to Killer Frost than ever before, but she remains hesitant to truly hurt Barry or Cisco, and even gets herself knocked out at the end of the episode, causing some to wonder, is Caitlin still Killer Frost on The Flash? The answer, it might surprise you to find out, is that Caitlin was never really Killer Frost — at least not yet.
In "I Know Who You Are," Caitlin goes up against Team Flash for the first time and purposefully hurts Cisco and Barry while doing Savitar's bidding. She even tries to kill Tracy Brand, an innocent woman, for the so-called Speed God. However, as is later pointed out by Savitar himself, there's one very important thing Caitlin hasn't done: she hasn't killed anyone. And, by the end of the episode, that hasn't changed. Not only has Caitlin not killed anyone (and thus not earned the villain name of Killer Frost), she also got knocked out during a big confrontation with Team Flash and was almost taken back to STAR Labs by Cisco and Julian.
Before Savitar grabs an unconscious Caitlin and runs away, Cisco manages to get a sample of her Killer Frost blood. She's not a super villain yet, but biologically Caitlin is now the metahuman that becomes Killer Frost, which means that the team needs a sample of her DNA if they want a chance at saving Caitlin. Savitar still has Caitlin's physical body, but with Cisco and Julian working on a cure, it's only a matter of time before Killer Frost is gone, leaving Caitlin Snow safe and sound. At least, I hope it is.