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Killer Frost has been a popular addition to The Flash ever since she first appeared as Caitlin Snow's Earth-2 doppelganger back in season 2, but tonight's episode may have just changed her origins by revealing she's actually been around a lot longer than we thought - at least in this timeline, anyway.
When The Flash began, Caitlin Snow didn't have meta-powers, but then came "Flashpoint" and all the changes caused by Barry's fiddling with the timeline. When all was said and done, Caitlin came out of the deal with meta-powers. Presumed as a latent development from the particle accelerator explosion, Caitlin's newly developing powers were the very same cryokinesis powers of her Earth-2 doppelganger, Killer Frost. And over fear she'd soon become twisted and evil just like Frost, Caitlin was desperate for a cure. That cure was eventually discovered by Julian but there were side-effects, and as a result, Caitlin manifested a Killer Frost-split personality whenever she used her powers.
Caitlin Snow and Killer Frost have been in this Dr. Jekyll and Mr. Hyde state ever since, with season 4 seeing the two come to terms with their shared existence and grow closer. That is, until DeVoe deactivated Caitlin's meta-abilities and Killer Frost disappeared. Determined to reactivate her meta-powers and bring Killer Frost back, tonight's episode sees Caitlin dig deep into her past where she learns a shocking revelation.
There isn't a whole lot known about Caitlin's backstory before joining S.T.A.R. Labs, but in both the before and after "Flashpoint" timelines, her father died from an illness and to cope, her mother threw herself into her work, ignoring Caitlin. That backstory is recapped again in tonight's episode, but it's also suggested that Caitlin is repressing something about her childhood. Later on, while training with Barry and Cisco for their Flashtime mission to stop DeVoe, she collides with the wall as super-speed and suffers a concussion. While knocked out, Caitlin experiences a momentary flash of a childhood memory she doesn't recall, later asking Cisco if he would vibe her into that memory so she can work out what it means.
Caitlin's repressed memory turns out to be an incident from her childhood where she's hit by a truck while riding her bicycle. As she's lying on the pavement, twisted up in her bike, a young Caitlin reaches for the truck's broken side mirror and sees her reflection - white hair, white eyes. This means Caitlin was been a meta-human long before the particle accelerator exploded and even before Team Flash traveled to Earth-2. In fact, it appears that Caitlin's been suppressing her abilities along with her memories of having them for years, which means that in this timeline, Caitlin may have always been a meta-human.
What this means for Caitlin's efforts to reactivate her meta-powers isn't exactly clear, but this revelation does seem to imply there must be some way for Caitlin to get her powers back, and maybe her friend, too. What it also implies is that Caitlin must have come into contact with dark matter as a child since it's been established that dark matter is what turns a regular human into a meta-human. Could she have been affected at or just prior to her birth? Could the dark matter be connected with her father's death? Or perhaps her mother's work? Did this incident with the bicycle lead Caitlin to hide her powers (even from herself), creating the need for her powers to develop their own, split personality?
Answers to all these questions are more likely to come next season rather than in next week's season 4 finale, what with Team Flash needing to thwart DeVoe's Enlightenment before the entire world is struck dumb. But this little tweak in Killer Frost's backstory suggests that not only do we have a lot more to learn about Caitlin, but there may yet be a lot more to learn about meta-humans on the whole.