Earth-1's Killer Frost
Youth and aiding Team Flash
Both of Caitlin's parents were doctors, which instilled a great respect for science in her and eventually led to her becoming a doctor and scientist as well. After Caitlin's father died from multiple sclerosis, her mother completely devoted herself to her work as a way of coping with her grief. This led her to somewhat neglecting Caitlin which led to the two of them becoming estranged over the following years.
After becoming a doctor like her parents and turning into a skilled neurosurgeron, Caitlin was offered a job at her mother's company but she refused, intending to make a name for herself instead of hiding behind the name of her mother. Instead, she accepted a work offer from Harrison Wells and started working at S.T.A.R. Labs in Central City. There, she developed a close friendship with her co-workers Cisco Ramon and Ronnie Raymond, eventually falling in love with the latter. The two started dating. While at S.T.A.R. Labs, she also developed a bond with the lab gorilla Grodd, who was experimented on by the military on orders of General Eiling.
The night the S.T.A.R. Labs particle accelerator was to be switched on, Caitlin was present at the Lab together with her co-workers. When it was realized that something was wrong, Ronnie and Cisco headed downstairs to save them all but when the accelerator exploded, Ronnie was seemingly killed. Although they did not realize it back then, Caitlin and Cisco both became meta-humans during the incident. When Barry Allen was struck by the explosion as well, rendering him comatose, he was brought to S.T.A.R. Labs where Cisco, Caitlin and Wells tended to him.
After Barry regained consciousness and realized that he became a speedster, Caitlin played an extremely important part in the creation of Team Flash and continuously helped Barry and the rest of his team during their mission. She and Barry became close friends. During their adventures, the team also found out that Ronnie was still alive and they managed to recover him and to bring him back to the team. However, when the team later had to destroy the wormhole created in the final battle with Eobard Thawne, Ronnie was killed for good - sending Caitlin into a deep depression as she lost the man she loved twice.
Caitlin is only driven out of her depression when Jay Garrick arrives from Earth-2, warning Barry and the team of a sinister speedster called Zoom. Jay starts working together with the team and eventually, he and Caitlin fall in love. When Caitlin learns that Jay is dying from a sickness created by the speed-serum Velocity-9, she and the team promise to save him.
However, when part of the team visited Earth-2 to free the daughter of their team-member Harrison Wells from Zoom's lair, they are confronted by some of Zoom's meta-human mercenaries - Earth-2's Killer Frost among them. To Caitlin's horror, however, Jay is seemingly killed by Zoom when the team returns to Earth-1. This drives Caitlin into depression once more and hardens her against all her teammate's efforts to help. Caitlin realizes that everybody is behaving strangely around her and eventually learns of the existence of her evil-counterpart. However, she promises the team that she would never turn into something like Killer Frost.
Eventually, the team finds out that Jay Garrick and Zoom are the same person and that Zoom was using the team the entire time. Caitlin at first does not believe it but then decides that Jay, whatever his true name, needs to be taken down. Meanwhile, Zoom started preparations for invading and conquering Earth-1. However, he still had feelings for Caitlin which eventually led him to kidnapping her and imprisoning in his lair. During her captivity, Caitlin met her Earth-2 counterpart for the first time, as Killer Frost had been imprisoned by Zoom as well. During the imprisonment, Killer Frost tried to murder Caitlin but was killed by Zoom before she could do so. Eventually, while bringing his meta-human army to Earth-1, Zoom gave Caitlin a choice. She could either remain with him or return to her friends, making her Zoom's enemy. Caitlin decided to return to her friends. Caitlin proved essential in Zoom's downfall, as she managed to manipulate his feelings for her to lure him to the final standoff with the Flash.
According to Craig, a member of Savitar's cult, Caitlin will be described as being "glorious and powerful", and Savitar will have "special plans" for her.
In a news report on a television reporting the arrest of Jared Morillo that Barry saw during his accidental trip to May 2017, a subheading reads "Killer Frost still at large". While Barry succeeded in altering the circumstances of Morillo's arrest, with Wally defeating him instead of Barry in January instead, Caitlin's future remains the same. Barry later reveals to Joe that Caitlin's on the loose and they need to bring her in, prompting him to alert the CCPD. This causes the aforementioned headline.
When Barry visited a possible future, in the year 2024, it is revealed that Caitlin allied herself with Savitar. Moreover, during the aforementioned battle, Vibe's arms were frozen and shattered by Caitlin, forcing him to replace them with bionic arms and subsequently losing his powers. Caitlin, still as Killer Frost, has been locked up in the Pipeline, and still refuses to divulge the identity of Savitar, with Julian as her caretaker.
Becoming Killer Frost
After the Flashpoint-Timeline is dissolved, Caitlin suddenly starts developing symptoms of her meta-human powers. Having witnessed the cold-hearted Killer Frost first hand, Caitlin becomes deeply afraid that she will turn into an equally evil person with no emotions or feelings. Thus, she keeps her powers hidden even from Team Flash and starts searching for a cure on her own. However, when Mirror Master traps Barry in a mirror, Caitlin uses her frost powers to free him from it.
In her desperation, Caitlin even visits her mother and asks for help. She eventually snaps when she cannot stand her mother's distant behaviour anymore, freezing her desk and thus revealing her powers. Caitlin's mother immediately clears her schedule and the two Snow's start running tests to find a way of saving Caitlin. During that time, the two also solve out some of their issues. When Nigel, one of the lab assistants tries to prevent Caitlin from leaving the premises, intending to use Caitlin's powers for his own benefit, Caitlin coldly freezes his arm solid. With Killer Frost briefly taking over, Caitlin is ready to kill Nigel but her mother manages to appeal to Caitlin and talk her out of it. Caitlin's mother later contacts Caitlin again, revealing that Caitlin's powers will develop faster the more she uses her powers. Caitlin steals power dampeners from S.T.A.R. Labs in order to prevent her powers from spreading but Cisco eventually finds out and Caitlin admits her powers to him. Cisco makes Caitlin confess her powers to the entire Team Flash despite her not being ready for it.
In her fear of becoming Killer Frost, Caitlin ironically becomes more and more like her. When one of Savitar's acolytes is taken by the SCPD, Caitlin, now taken over by Killer Frost, storms into the interrogation chamber and tortures the man with her powers, demanding to know how to find Savitar as she believes that Savitar can take her powers from her. While leaving the precinct, Killer Frost is confronted by Julian Albert whom she swiftly kidnaps in order to force him to locate Savitar and the followers of Alchemy. Julian is able to do so but also alerts the GCPD, prompting Killer Frost to attempt to kill him. However, Barry arrives in time to confront Killer Frost. When Cisco arrives as well, Killer Frost attacks the two, kissing and thereby freezing Barry completely who is saved just in time by a vibe from Cisco. The vibe also knocks out Killer Frost who is then imprisoned in the pipeline. After regaining consciousness, Killer Frost attempts to convince Barry and the team that she is back to normal but this fails. Instead of abandoning Caitlin, however, Barry refuses to give her up and eventually opens her cell, telling her that she would have to kill him if she intends to get out of S.T.A.R. Labs. Although Killer Frost creates an icicle, she is unable to kill her friend and returns to her Caitlin-form in tears.
Attack on S.T.A.R Labs
A mysterious future villain of the Flash named Abra Kadabra appears, stealing from several tech companies in an attempt to build a timeship to return to the 64th century. Abra Kadabra soon finds himself chased after by the Flash and Gypsy, escaping, but not before telling Flash that he knows Savitar's name. Sometime later, he teleports into S.T.A.R Labs and ambushes Caitlin, Julian and Joe West, but is soon apprehended by the Flash and locked up in the Pipeline where he keeps taunting the team about his knowledge of Savitar.
He tells Joe that if he lets him escape from Earth-19 bounty hunter Gypsy, he will reveal Savitar's name so the team can save Iris. Joe agrees, and sets him free, but Kadabra betrays him, claiming that the terms of the agreement have changed. He then collects a mysterious future power source from the Time Vault, and sets off an explosive as Caitlin, Joe and Julian attempt to chase after him upon his escape. The resulting explosion impales Caitlin's side with shrapnel. To evade bringing Caitlin to a hospital where she would be exposed as a meta-human, Caitlin is forced to supervise her own surgery. Initially, all goes well and Caitlin falls asleep after the shrapnel is successfully removed. Later however, her heart rate suddenly spikes,, putting her life at severe risk. Torn between respecting Caitlin's wishes that she'd rather die than become Killer Frost, or to tear off the power-dampening pendant to possibly bring her back, Julian ultimately decides to tear the pendant off, which results in Caitlin's revival power healing her at an enormous speed. Caitlin's wound heal instantly, but she also emanates a cloud of mist, sending the team and herself flying. Once the cloud clears, Caitlin emerges from the mist fully transformed into Killer Frost and attacks the team.
Soon after her escape from S.T.A.R Labs, Caitlin is wandering around in the snow and finds Savitar. She then asks if Savitar wants to cure her, to bring Caitlin Snow back, to which he replies that he wants the opposite. Caitlin asks why she should trust him and Savitar removes his armor, which causes her to state she'll do anything he wants.
Once Killer Frost has sworn herself to Savitar, Savitar informs her that scientist Tracy Brand will develop the device that will trap Savitar in the speed force four years from the present. He tells her to kill Brand so that she can never create the device. Thus, Killer Frost heads to the Central City university where Brand is currently located. However, when she storms the university, she finds that Barry, Cisco and H.R. have already turned up to recruit Brand. Killer Frost hurls an ice blast at Brand but Barry jumps in front of it, taking the whole blast. This leaves only Killer Frost and Cisco standing. Cisco tells Killer Frost that he does not want to hurt her and Killer Frost replies that she knows he doesn't. However, she adds that she will and shoots more ice at Cisco, who is taken by surprise and blasted backwards. Meanwhile, Barry gets back up and uses the laboratory equipment to create a small explosion directed at Killer Frost. Driven backwards by the heat, Killer Frost is forced to flee.
She returns to Savitar's lair and informs him that she failed in killing Brand because Cisco and Barry interfered. Savitar is surprised, as he had informed Killer Frost that the two would be present. He wonders whether there is still part of Caitlin in Killer Frost and then reveals to her the importance of Brand's death. He states that the only thing standing between the two of them and true "ascension" to a level above pain and hate is Tracy Brand. He once more tells her that Brand must be eliminated and Killer Frost sets off to murder Brand.
Killer Frost tracks down Brand to the coffee store Jitters. There, Brand is together with H.R. but the two are warned about Frost's arrival when the coffee freezes inside their mugs. They run outside and call the nearby-posted Barry for help. The team assembles around them, just in time to witness Killer Frost arriving. H.R. and Brand run into the city to escape but Killer Frost pursues them by creating a frozen slideway in front of her feet - swiftly speeding after the two high above Central City's streets. Barry, in turn, pursues her and manages to direct her back to Jitters where Cisco is waiting. However, Cisco once again remembers that his friend is still stuck inside Killer Frost and that he might accidentally kill her if he attacks her. Thus, Cisco refrains from attacking Killer Frost and, sensing his weakness, Frost blasts another stream of ice at him. Once more, Barry jumps in front of the target and is hit by the blast and thrown down a stairway. Frost slowly walks down the stairway and tells him that Savitar forbade her to kill Barry. However, claiming that she cannot have him hunt her down, she creates an icicle which she rams into Barry's leg to temporarily slow him down.
Fed up with having to hunt down Tracy Brand, Killer Frost kidnaps Joe's love interest Cecile instead. With Cecile as her hostage, Killer Frost turns up at the West house and warns Joe that Cecile will die should Brand not be surrendered to her. She informs Joe that he will receive a message with an adress shortly. When Joe attempts to stop Killer Frost from leaving with Cecile, Killer Frost uses her ice powers on him.
An hour later, Barry and the team turn up at the address Killer Frost sent them. Secretly, Cisco heads up into the rafters in order to later ambush Killer Frost. When Killer Frost reveals herself, Barry tries to appeal to Killer Frost but surprisingly, Killer Frost already knows what exactly Barry will say and keeps saying the words together with him. After Barry stops talking, Killer Frost laughs a humorless laugh and reveals that Savitar told her exactly what Barry would say to her. She adds that Savitar also told her that Cisco would be hiding in the rafters and activates hidden explosives that blast Cisco off the roof. She then hurls another icicle at Brand which, once more, Barry takes. Once Barry has gone to the floor, Killer Frost freezes his torso in order to prevent him from getting back up. However, she is then hit in the back by Cisco's vibe powers. She quickly gets back up and starts a battle with Cisco, who has finally overcome his fear. Both blast their powers against each other and Cisco eventually gains the upper hand and blasts Killer Frost back several feet. After making sure she is still alive, Cisco takes a blood sample from Killer Frost but before he can do more, Savitar speeds into the room and grabs the unconscious Killer Frost. He then speeds off and takes Caitlin with him.
After Savitar reveals his identity to Barry, he is the Future Flash, Killer Frost returns to Savitar's lair in order to find out what they next plans are. However, as Savitar is the Future Flash, Barry and Team Flash intend to stop Barry from remembering things so that Savitar cannot use them against him. While attempting to do so, they accidentally wipe Barry's memory completely which also affects Savitar, who neither remembers his ally Killer Frost nor his own identity. Shocked by the intruder, Savitar grabs Killer Frost and smashes her into a wall.
Realizing the problem, Killer Frost walks into S.T.A.R. Labs. She claims that Barry's loss of memory is not only a problem for them and proposes to work together in order to solve it. When Wally tries to stop her, he realizes that he has no speed. Killer Frost replies that she knows exactly why this is the case and promises to explain should the team follow her plan. Back at S.T.A.R. Labs, Killer Frost takes control over the technology in order to program the machinery so that Barry can get his memories back. She is working together with Julian and Cisco and Cisco attempts to get the old Caitlyn back by telling Julian some of their past adventures together. Although Killer Frost coldly dismisses the story, she is visibly touched. After succeeding in bringing back both Savitar's and Barry's memories, Killer Frost turns to leave. Although they try to convince her to stay, Team Flash does not fight her. Julian tries hardest to convince her, claiming that he will find her a cure and telling her that he loves her. However, Killer Frost replies that she never loved any of them and enters the elevator. Inside the elevator, she turns back into Caitlyn for a second before immediately turning back into Killer Frost.
While Team Flash does anything in their powers to build the weapon that will trap Savitar in the Speed Force, Savitar himself has some problems as his suit has been damaged by Barry during their fight. When Killer Frost realizes this, she mocks him about not looking very god-like at the moment. When she states that the two of them are a team, Barry and Caitlyn back at it again, Savitar stops her and claims that she is not Caitlyn and that he is not Barry. Killer Frost then asks whether Savitar will actually go through with it and kill Iris. Savitar claims that it is either her or him; if Iris doesn't die, Savitar will never be born. Killer Frost mentions that Savitar still has all memories of Barry and thus remembers every memory Barry has with Caitlyn. Savitar responds by claiming that he also remembers being created as a disposable hero, that Barry did not care for his life and that he thus does not care for the life of Barry's fiancee. The same moment, Savitar obtains memories from the Flash, his past identity, and realizes that Barry has travelled through time again and has recruited Leonard Snart to help them steal the energy source for their speed cannon. He is not concerned but only amused and tells Killer Frost that she doesn't have to intervene as it will not make a difference.
While waiting for the time of Iris' death to come, Savitar and Killer Frost do not realize that H.R. has tracked down their hideout. While the two are distracted with Savitar's armor, H.R. frees Iris but the part of Savitar's blade H.R. used to track him with suddenly snaps back into the armor, alerting Savitar who orders Killer Frost to deal with the intruders. Using technology from his Earth, H.R. then reversed the identities of him and Iris. When Killer Frost arrives and blasts what she believes to be H.R. away with her powers, she actually blasts away Iris and recaptures H.R., who now looks like Iris.
Fifty minutes before the moment Savitar kills "Iris", Cisco suddenly has a vibe of Killer Frost challenging him inside a forest. Although he realizes that they are planning to separate the team, Cisco, with Barry's blessing, heads out to confront Caitlyn and to save her from herself. When he meets Killer Frost, she reveals that she has to kill Cisco in order to fully kill Caitlyn. She then blasts icicles at Cisco, which Cisco can blast away at first. However, she eventually hits him. Before she can finish him off, however, Savitar speeds in and tells her to spare Cisco as Cisco has to build something for him.
Savitar has Killer Frost freeze Black Flash, which she does gladly after the suffering Zoom brought to her. Jay Garrick is released from the Speed Force and most of the heroes come to fight them. After neutralizing her, Cisco gives her a serum to delete her powers. When Savitar is going to kill Cisco, Caitlin betrays Savitar and throws an ice blast to stop him.