There have been several iterations of the Joker character in various DC comics, films, and television shows. However, the most well-known Robin who becomes the Joker is Jason Todd, the second Robin who was introduced in 1983.
Jason Todd was originally a street orphan who attempted to steal the tires off the Batmobile. Batman took him under his wing and trained him to become the new Robin, succeeding Dick Grayson who had moved on to become Nightwing. However, Jason was often more reckless and impulsive than Dick, and he frequently clashed with Batman over their methods.
In the 1988 storyline “A Death in the Family,” Jason Todd’s fate was sealed. In this story, the Joker discovered that Robin’s secret identity was Jason Todd and proceeded to kidnap him. The Joker brutally beat Jason with a crowbar, and then left him and his birth mother in a warehouse rigged with explosives. The bomb went off, killing both Jason and his mother. The story ended with Batman holding Jason’s lifeless body in his arms.
However, years later, in the 2005 storyline “Under the Hood,” Jason Todd was revealed to have been resurrected by Ra’s al Ghul using the Lazarus Pit. Upon his return, Jason was consumed with anger and resentment towards Batman for failing to avenge his death and for not being there to save him. He donned a new persona, the Red Hood, and embarked on a violent and deadly crusade against crime in Gotham City.
Eventually, in the 2010 storyline “Batman: The Return of Bruce Wayne,” Jason is revealed to have been manipulated by Darkseid into becoming a pawn in his plan to destroy Batman. This revelation causes Jason to have a mental breakdown and he becomes the Joker. As the Joker, he is even more sadistic and twisted than before, and he launches a brutal campaign of terror against Batman and the citizens of Gotham.
It is worth noting that this storyline is not universally accepted as canon, and there are many other versions of the Joker’s origin story. However, the idea of Jason Todd becoming the Joker has become a popular fan theory, and it has been explored in various comics and media adaptations.
In conclusion, Jason Todd is the most well-known Robin who becomes the Joker. However, the circumstances surrounding his transformation into the Joker vary depending on the version of the story. Whether or not this storyline is considered canon, the idea of a former Robin becoming the Joker adds an interesting layer to the complex relationship between Batman and his sidekicks.