Give a stream of consciousness and then the final answer. The character was introduced in Batman # 1 (1940), in which he announces that he will kill three of Gotham's prominent citizens (including Mayor Henry Claridge). Although the police protect Claridge, the Joker had poisoned him before making his announcement and Claridge dies with a ghastly grin on his face; Batman eventually defeats him, sending him to prison. The Joker commits whimsical, brutal crimes for reasons that, in Batman's words, "make sense to him alone". Detective Comics # 168 (1951) introduced the Joker's first origin story as Red Hood: a criminal who, during his final heist, vanishes after leaping into a vat of chemicals to escape Batman. His resulting disfigurement led him to adopt the name "Joker", from the playing card figure he came to resemble. The Joker's Silver-Age transformation into a figure of fun was established in 1952's "The Joker's Millions". In this story the Joker is obsessed with maintaining his illusion of wealth and celebrity as a criminal folk hero, afraid to let Gotham's citizens know that he is penniless and was tricked out of his fortune. The 1970s redefined the character as a homicidal psychopath. "The Joker's Five-Way Revenge" has the Joker taking violent revenge on the former gang members who betrayed him; in "The Laughing Fish" the character chemically adds his face to Gotham's fish (hoping to profit from a copyright), killing bureaucrats who stand in his way.

Based on this passage, who was the joker before becoming the joker?
The relevant information to answer the above question is: Detective Comics # 168 (1951) introduced the Joker's first origin story as Red Hood: a criminal who, during his final heist, vanishes after leaping into a vat of chemicals to escape Batman.
So, the final answer is Red Hood : a criminal who , during his final heist , vanishes after leaping into a vat of chemicals to escape Batman.