Arthur Fleck was born on November 21, 1946. 35 years later, Arthur was very lonely and socially awkward with only his mother for real company besides a pet cat. His psychological suffering was compounded by his pathological laughing disorder, often causing him to laugh at inopportune times. At one point his mother had an abusive boyfriend some years after Arthur's birth, who reportedly beat both him and his mother, causing his nervous tick. At some point, he became a party clown where he met fellow co-workers Gary and Randall.
- Imaginative - Imagines scenarios when he zones out, usually of happier possibilities
- Smokes often
- Uncontrollable laughter
- Passion for dancing
- looking for an emotional connection
- Penny Fleck - Mother and victim; deceased.
- Sophie - Neighbor.
- Murray Franklin - Idol and victim; deceased.
- Thomas Wayne - Supposed father; deceased.
- Bruce Wayne - Suspected brother.
- Gary - Former co-worker.
- Randall - Former co-worker and victim; deceased.
Behind the scenes
- Joaquin Phoenix lost 52 pounds for the role.
- In true Joker fashion, it is shown throughout the movie that Arthur is an unreliable narrator, as it is revealed his relationship with his neighbor was completely imagined.
- Arthur may in fact be the illegitimate son of Thomas Wayne, making him Bruce's half-brother. Despite the documents stating he was adopted and his mother was a liar, Arthur found a picture of his mother young with Thomas Wayne writing that he loved her smile. This theory is also backed by Alfred's reaction to Arthur showing up, pushing Bruce out of earshot and whispering to Arthur 'nothing happened.' This, along with Penny's statement that she had signed papers to keep quiet, fit with Thomas' character of being self serving.