- "My once promising career in bio-engineering is now over, my boss is in a wheelchair for life. The explosion that put you in a coma also killed my fiancé, so this blank expression feels like the way to go."
- —Caitlin to Barry Allen on her general lack of smiling[src]
Dr. Caitlin Snow is a bio-engineer at S.T.A.R. Labs. She was working during the particle accelerator disaster and tended to Barry Allen during his coma with Cisco Ramon until he woke up. After he began crusade as "The Flash" Caitlin became a member of his team with Cisco and Dr. Harrison Wells and provides medical support for the team. Caitlin is also the fiancé of Ronnie Raymond, and a good friend of Cisco, Barry, and Felicity Smoak.
Caitlin was working on the night that the S.T.A.R. Labs particle accelerator was to be switched on. She and her fiancé, Ronnie, were discussing their honeymoon when they were alerted of the impending switching on. After he switched it on, Dr. Harrison Wells went to make a toast, but it was made apparent that the particle accelerator was malfunctioning. Being the structural engineer, Ronnie had to go down with Cisco Ramon to try to, if not repair it, contain the blast. Despite her hesitation, he had to go. After he didn't come back in a while, Caitlin went down to talk to him. However, Ronnie had had Cisco put the accelerator in to lockdown, locking him inside the core chamber. Caitlin and Ronnie talked, but before they could continue, Ronnie's connection was lost, presumably being killed inside the chamber, greatly upsetting Caitlin.
Following an accident involving lightning striking him, Barry Allen was put in a coma. After his situation got critical, he was moved to S.T.A.R. Labs where he was stabilised. Sometime following his move, Cisco, along with Caitlin, met Felicity Smoak during one of her visits to see the patient.
Time in Starling City
A little while later, Caitlin and Cisco were asked to pack up a S.T.A.R. Labs warehouse in Starling City, following the massive explosion that occurred in Central City. The two were doing an inventory count, discussing the particle accelerator's explosion, when they heard movement in the warehouse. They went to investigate, only to find a security guard standing there. Cisco assured him that they were almost done, though he just stared them down, creeping him out. Before the man could reply, he spat blood out and dropped to the ground, with Slade Wilson/Deathstroke ready behind him. The two made a run for it, managing to incapacitate Deathstroke momentarily with an old energy gun made by Arthur Light. They later reported the incident. Sometime after, they met Felicity Smoak and John Diggle at the hospital. Felicity asked about Barry's condition, and Caitlin told her how it was still the same. Cisco quickly blurted out that "Iris" had been visiting him a lot. When Felicity asked who Iris was, Cisco struggled to explain their relationship, causing Felicity to walk off distressed that "Barry's in a coma, he's already moved on". A little while later, Felicity met the two of them in a lab, with a vial of Mirakuru, telling them that she needed their help and that it would be their little secret. Caitlin and Cisco sucessfully created a cure to Mirakuru that would be used to cure the Mirakuru soldiers during Slade's seize of the city and cure Roy Harper of his unstable condition.
Working with The Flash
Nine months after he was struck by lightning, Barry awoke from his coma to Lady Gaga's "Poker Face", a song he allegedly liked. Cisco called Dr. Wells down to lab as quickly as possible, and explained to Barry what had happened, introducing he and Caitlin. The two took a break as Wells showed Barry around S.T.A.R. Labs, explaining in depth what had actually happened. Later, after discovering that he had the ability to run super fast, the team of four headed down to an airfield. After Barry commented on her lack of smiling, she admitted that it felt like the best course of action considering what she'd been through. As Cisco monitored Barry's speed, Caitlin measured his energy output, amazed at how fast he could run, before he tripped over in to barrels of water. They headed back to S.T.A.R. Labs where Wells and Barry discussed the potential for the existence of other people like him, specifically another who had super speed like Barry, though Harrison was certain that Barry was unique. He left to meet Iris. After an encounter with Clyde Mardon's weather manipulation, he returned to S.T.A.R. Labs where he was angry at Wells for lying to him in regards to metahuman abilities. Barry returned later, sorry, offering an apology in return for Cisco and Caitlin's help in catching rogue metahumans. They agreed to help, and Cisco revealed a suit he'd designed for use by firefighters, which he believed would suit Barry's purposes well. Barry donned the suit and headed out to where they pinpointed Mardon to be. Cisco and Caitlin communicated with Barry, giving support. When Barry began to attempt to unravel it, Cisco stated his belief that Barry could withstand the pressure, though Caitlin didn't believe Barry himself would. However, after motivation from Dr. Wells, he eventually did.
After a run-in with a strongman at the Central City National Bank, Barry arrived back at S.T.A.R. Labs with a wrongly-healed ankle, after having previously broken it. They decided to re-break it, and Caitlin suggested to run extra tests to make sure that there was no further damage. Following that, Caitlin and Cisco tagged along with Barry to grab some DNA samples from the crime scene at which the strongman had been. While analysing the sample, they helped Barry find a group of chimpanzees that had escaped from Central City Zoo. As he locked them up, they revealed that the DNA sample was that of Joey Rose, a former strongman at the Central City Circus. They delved deeper in to the closed circus, discovering that its owner had escaped from Belle Reve and killed several people. They notified Barry of this, while Dr. Wells also provided some details he knew on Mr. Bliss from having attended the circus previously, and Barry left to find the circus. Having been gone for a while, Cisco and Caitlin took a truck and crashed through the tent of the circus. Cisco grabbed Barry and Caitlin attempted to drive away, however their truck was lifted up by Joey Rose, who threw it some distance from the circus tent. As they crawled from the wreckage, they encountered the snake lady. The lady promised to let them pass, but Caitlin believed it to be a trap. The lady assured her that she was genuine, as they were only compliant to Mr. Bliss otherwise torture would be inflicted on them. Before leaving, Caitlin asked what all of it was about, but the woman was only aware that something big was happening. They headed back to S.T.A.R. Labs, where Caitlin explained what had happened to Barry and Cisco further explained what the snake lady had told them.
Caitlin walked in on Cisco as he guided Barry towards a burning building to save people inside. Once he had saved everyone, she ordered him back to S.T.A.R. Labs. Though he tried to justify what he was doing, using his abilities for good, she mentioned that they had actually agreed to locate and contain metahumans together. Not being able to get her message across, she had Dr. Wells try to help communicate it. She angrily told Barry that she couldn't fix him every time something broke before walking off. After later fainting, Barry headed to S.T.A.R. Labs, where Caitlin yelled at Barry for keeping it to himself, as he could've suffered from a mini-stroke. They decided to test his vitals while he ran on a modified treadmill. They noted his low glucose levels, but not before he fell off and hit the padding. They hooked him up to 40 IV bags in order to bring him back to normal. When he awoke they explained what they determined to be the problem: he hadn't been eating enough for his massive metabolism. Shortly following, Joe appeared through the doors. He began to get angry at Barry for acting as a hero, and Caitlin agreed with his reasoning, before Joe left upset. Yet again Barry later returned to S.T.A.R. Labs, having had a run in with a metahuman who could duplicate himself, who they found to be Danton Black. From some of the perp's blood on Barry's suit, Caitlin was able to speed up the replicating process, making a clone from cells. The clone came to life, freaking Caitlin out, though Joe had returned, shooting the clone. They figured that Danton Black had activated some clones, thus the clone's movement, and Barry suited up to head to Stagg Industries. The team provided support to Barry as per usual, Caitlin suggesting to locate the prime in order to take them all out. Barry ultimately beat Danton, who fell to his death. They soon watched a report about it on the news. Barry assured that the three of them were always with him, and that really all of them had been struck by lightning, not just Barry, at least in the metaphorical sense.
A while later, Cisco and Caitlin alerted Barry to a man with a gun in a getaway car being chased by cops, who Barry sped to and placed in the cop car within a matter of seconds. The following day, after the Darbinyan crime family were gassed to death by a presumed metahuman, the S.T.A.R. Labs team began researching what they could, all of them excited by the potential prospects. Joe noted that they would have to find somewhere to hold them, as Iron Heights was less than ideal, and Cisco suggested the particle accelerator. Not wishing to go down, Barry suggested she tag along with him to identify the poison gas. At the CCPD, they headed upstairs. On the way they were joined by Barry's boss, Captain Singh. Caitlin claimed to be Barry's personal physician. Once they were in the lab waiting for the gas' test results, they discussed Caitlin's late fiancé Ronnie. Once they got the test results, they were intrigued to find two separate tissue samples in it, rather than just one, coming to the conclusion that the metahuman could in fact turn in to gas himself, hence why his genetic residue was left behind. Barry left to find the metahuman after hearing news on a police radio, leaving Caitlin at the lab. He eventually found the man, but inhaled some of the gas. He returned back to S.T.A.R. Labs where Caitlin used a needle to painfully extract the gas before it killed him. Once he woke up, Barry apologised for leaving so suddenly, but Caitlin reassured him that it was fine and he had just reminded her of Ronnie. Barry related the story of his mother's death, and offered to go in to the core chamber with Caitlin to conquer her fears. While they were there, he promised her that Ronnie was a true hero. They were called back up where they were shown the molecular makeup of the poison gas, hydrogen cyanide mixed with a sedative. Barry recalled that executions used those particular chemicals and Caitlin uncovered Kyle Nimbus, who would have been executed at the time of the particle accelerator explosion. She further went on to look up who caught him, Detective West, figuring that he was his final target. Caitlin gave him an antidote to the toxin, and Barry sped to Iron Heights Prison to protect Joe. The team supported Barry, and once he had lured Nimbus out in to the open, Caitlin reminded him not to breath any gas in (as he'd used the antidote on Joe). Finding it too difficult to outrun him, Caitlin suggested to simply have him continue running at Barry to sap his strength, forcing him to revert to human form. Finally capturing The Mist, as Cisco proudly nicknamed him, they put him in the makeshift particle accelerator prison they'd engineered. Cisco attempted to apologise for the night of Ronnie's death, however Caitlin admitted that she was okay, as she was even more proud of Ronnie. The two headed out to the cinemas, followed by ice cream.
Caitlin played Barry in a game of Operation while Barry was also played a game of Chess with Harrison Wells, and a game of Ping pong with Cisco while Barry was testing his ability to multitask. When Felicity arrived at S.T.A.R. Labs, Caitlin was pleased to see her again though she was worried that she would know to much of their operation. Caitlin was told that she wouldn't need to worry after being told Felicity works with The Arrow. After the cold gun was stolen, Caitlin along with Harrison learned that Cisco built a weapon that was powerful enough to stop Barry, despite his speed. After Barry was struck by the cold gun, Caitlin noted that Barry's nerve cells would've been damaged permanently had they not regenerated at the speed they did. When Cisco told Barry he built the cold gun to stop him, Caitlin defended Cisco that he was only taking precautions and that it was when Barry was still in a coma. Caitlin was later shocked when Felicity managed to track down Leonard Snart/Captain Cold in a matter of seconds. When Caitlin and Cisco was advised by Felicity to follow Barry and help him, Caitlin reminded Felicity that Barry wanted to be alone. Caitlin was told by Felicity that when someone asks to be alone, that is the best time to help them. Caitlin, along with Cisco and Felicity brought the S.T.A.R. Labs vacuum cleaner, claiming it was the prototype cold gun to threaten Snart.
Barry took Caitlin and Cisco to a social gathering with Iris and Eddie Thawne, who she expressed an attraction, but Barry told her he couldn't get drunk so she took a sample of his blood in a syringe, which she kept in her bag, but Barry was soon called to a bombing in a building where a man was hanging for his life. Later Barry went to find the bomber Sgt. Bette Sans Souci but returned moments later without his suit, after Caitlin desperately tried to get back in contact with him over communications, stating that Bette destroyed it causing it to explode with a single touch. Flash managed to bring Bette back to the lab where Caitlin was able to figure out that the dark matter of the particle accelerator merged with a piece of shrapnel where she got her powers, and later observed as Bette tested her powers. Bette told everyone that she was being targeted by her commanding officer Gen. Wade Eiling and Caitlin tried to find a way to remove her powers but discovered that there was no means of reversing it and though Barry wanted her to stay, become a member of the team, every one including Caitlin feared she would eventually unintentionally destroy the lab and they needed to avoid being Elling's enemy. After Bette was killed by Elling, Barry became slightly depressed and Caitlin tried to cheer him up by making note of his power to walk on water and later made an whisky shot strong enough for Barry to feel. 
As Caitlin and Cisco debate how many insects Barry swallows while running, Barry came to S.T.A.R labs with a broken hand caused by metahuman Tony Woodward, Barry's childhood bully, which Caitlin tended to but noted that 13 bones were broken in his hand alone among multiple injuries in his whole body. She later watched as Barry practiced fighting on a robotic dummy and dislocated his arm and Caitlin tried to fix it but when she asked if Barry was visiting Iris as the Flash again, he lied promoting Caitlin to fix his arm by brutally snapping it back in place with her hands. Caitlin and Cisco later deduced Tony's hiding place but when Barry was seriously wounded Caitlin and Cisco had to go find him themselves and bring him back to the lab. Cisco and Dr. Wells both agreed that Barry could defeat Tony if he ran over 800 miles but Caitlin begged him not to as it would shatter every bone in his body if done wrong. Later Barry engaged Tony again after he kidnapped Iris and both Caitlin and Cisco begged him to leave which he did, only he turned around to run at 800 miles which Caitlin begged him not to but nerveless he succeeded. Caitlin watched as Barry confronted Tony, face to face, and was amused by Cisco's desire to stop his own childhood nemesis next. 
Caitlin is very passionate about her work as a biologist, even keeping a syringe in her purse, and is very intelligent. She loved her fiance Ronnie Raymond very much and was devastated by his death which sent her into a state of depression for almost a year. However since being on the team she livened back up, especially after admitting her grief to Barry. She has shown great concern for Barry as she often advises against engaging metahumans they know little about and seems to be weary of Iris because of Barry's feelings for her. Because of this it's implied that she may have since formed affectionate feelings for Barry. This is also hinted by Dr. Wells and Cisco when she grew angry with Barry for risking his life, they stated that she's only acted like it around Ronnie. She is also good friends with Cisco but finds his need to nickname every metahuman they face annoying.
The Flash: Season Zero
- "The Strongman Cometh"
- "Who Let the Chimps Out?"
- "A Goodnight Hiss"
- "Big Top Brawl"
- "A Captive Audience"
Behind the scenes
- Caitlin Snow is better known from DC Comics as the fourth iteration of the super-villain known as Killer Frost.
- Caitlin says in "" that Ronnie referred to them as a couple as "fire and ice", a reference to their respective DC comic identities.
- Furthermore Caitlin is often seen wearing blue, another possible reference to her super-villain name.
- It has been implied that Caitlin may harbor feelings for Barry, as she was multiple times shown great concern for Barry's safety and often advises him not to fight metahumans with risky methods and was even against his crusade to fight criminals.