Frost

1999: Father experimented on Caitlin Snow to cure her ALS;
It worked but she began exhibiting a dual personality that she was suppressed
2016: Persona started emerge periodically, as her meta-human powers emerged
Became a full-blown split personality, the actions of whom Caitlin would not remember
Lived a life of crime as Killer Frost, sought redemption, joined Team Flash, as Frost
Shared Caitlin’s body for years, appeared whenever Caitlin was scared or angry
Exposed to Mirror Monarch's powers, cells replicated, gained a separate body
Became Hellfrost to save Caitlin & team Flash from Deathstorm
2022: Absorbed Deathstorm into herself, killing her in the process