Frost was Caitlin Snow's alternate personality. Since she was a young child, Frost existed within Caitlin due to cryogenic experiments performed on the latter by her father, Thomas Snow. However, she was suppressed until Caitlin’s time with Team Flash, when her alternate personality began to remerge, as Killer Frost. Originally, Caitlin's Killer Frost powers mostly amplified her insecurities and anger, changing her chemical balance, but the further she continued to use these powers, the further she changed.

Eventually, after defeating Savitar, Caitlin developed a special partnership with her Killer Frost side, going on to analyze her condition, which resulted in Killer Frost becoming a full-blown split personality, the actions of whom Caitlin would largely not remember. Frost gave up her life of crime, dropped the “Killer” from her name and actively helped team Flash.

Cisco later gave Caitlin a mental activity dampener so she could communicate with Killer Frost. Soon after defeating Mirror Monarch, both Frost and Caitlin suffered from immense recurring headaches, which seemingly led to them becoming separated from one another and Frost inhabiting a body of her own. Soon after defeating Mirror Monarch, both Frost and Caitlin suffered from immense recurring headaches, which seemingly led to them becoming separated from one another and Frost inhabiting a body of her own.

Eventually, Frost became Hellfrost and confronted Deathstorm with her newly upgraded powers. After the two fought throughout the city, Frost absorbed Deathstorm into herself, killing her in the process.