The Joker

The Joker (legal name: Arthur Fleck), otherwise known by his previous persona Red Hood, is a crime lord in Gotham. Born to the omega Penny Fleck in the Gotham borough Edgewater, otherwise known as the Narrows, he lived with his mother in 1984. After that, he was human trafficked as a prostitute under the control of the Penguin, until some point before 1992, when he became known as the streets as Red Hood and started his own gang, which became the most prolific gang in Edgewater until January, 2003. He was not known as an omega after he escaped the Penguin, as he chose to keep his designation a secret.
In 1997, still as Red Hood, he killed the biological parents of Dick Grayson and held him captive for a week, until he was rescued by Batman. Red Hood subsequently became aware that Dick had been adopted by Bruce Wayne and Tony Stark, but did not make the connection between Batman and Bruce Wayne as them being the same person.
In January 2003, during a skirmish with the Batman, Catwoman, and Black Canary, while still in the identity of Red Hood he fell into a vat of acid. This was assumed to have killed him; however, it is now known that that was not the case, and he is still alive. The chemicals did, however, cause enhancements in him as well as other physical effects, including the bleaching of his skin and brain damage. Partially as revenge for this, the Joker started a reign of terror against Gotham in March and April of 2003, which led to him dunking Batman's childhood friends, Harvey Dent and Rachel Dawes, in vats of chemicals similar to the ones that affected him. Harvey subsequently died. The Joker killed several more people outside of him, up until he was stopped by Batman. He has now been committed to Arkham Asylum, being declared unfit for trial and medically insane.