The Joker is a supervillain in the DC Comics universe and the archenemy of Batman. His origin is famously ambiguous. The most common backstory (popularized by "The Killing Joke") involves him being a failed comedian who falls into a vat of chemicals, bleaching his skin and driving him insane. However, the Joker himself says, "If I'm going to have a past, I prefer it to be multiple choice," emphasizing his chaotic and unreliable nature.

In the 2019 film "Joker," Joaquin Phoenix plays Arthur Fleck, a mentally ill, impoverished stand-up comedian disregarded by society. This version is a grounded, psychological character study devoid of the usual superhero tropes (no chemical vats). It explores how systemic neglect and abuse can transform a vulnerable man into a symbol of nihilistic chaos. Phoenix won an Academy Award for this portrayal.

The Batman-Joker dynamic is one of the most complex in fiction. They represent order vs. chaos. The Joker is often depicted as being obsessed with Batman, believing they complete each other. He doesn't necessarily want to kill Batman but wants to break him morally—to prove that "one bad day" can turn even the noblest hero into a madman like himself.

Harley Quinn (Dr. Harleen Quinzel) was originally the Joker's psychiatrist at Arkham Asylum who fell in love with him and became his sidekick and accomplice. Their relationship is typically depicted as abusive and manipulative on the Joker's part. In recent years (both in comics and movies like "Birds of Prey"), Harley has broken free from his control to become an anti-hero in her own right.

The Joker has been played by several legendary actors: Cesar Romero (1960s TV series), Jack Nicholson (1989 Batman), Heath Ledger (The Dark Knight, winning a posthumous Oscar), Jared Leto (Suicide Squad), Joaquin Phoenix (Joker, Joker: Folie à Deux), and Barry Keoghan (The Batman). Mark Hamill is the definitive voice actor for the character in animation and video games.

Joker Venom is a signature weapon used by the Joker. It is a deadly toxin (liquid or gas) that causes its victims to laugh uncontrollably and eventually die with a grotesque, fixed grin on their faces (rictus grin). He has used various strains of this toxin over the years to terrorize Gotham City and kill his targets in a signature theatrical manner.

In Christopher Nolan's "The Dark Knight," Heath Ledger's Joker has "Glasgow smile" scars. He tells conflicting stories about how he got them—one involving an abusive father, another involving self-mutilation for his wife. This narrative inconsistency reinforces his mysterious, chaotic nature; the audience never learns the truth, making him more terrifying as an agent of chaos without a clear motive.

In most storylines, the Joker is considered legally insane and is sent to Arkham Asylum rather than Blackgate Penitentiary or executed. However, some writers (like Grant Morrison) suggest he possesses a form of "super-sanity," reinventing his personality daily to cope with the chaotic modern world. He is aware of his actions but lacks moral conscience or empathy.

The "Three Jokers" is a DC Comics storyline that reveals there have actually been three different men operating as the Joker over the years: The Criminal (Golden Age), The Clown (Silver Age), and The Comedian (Modern Age). This plot point attempted to explain the character's vastly different personalities and methods over decades of comic book history.

"The Killing Joke" by Alan Moore is a seminal graphic novel where the Joker attempts to drive Commissioner Gordon insane to prove a point to Batman. He shoots and paralyzes Barbara Gordon (Batgirl), then tortures her father. It is a dark exploration of the Joker's psychology and contains the famous "multiple choice" origin story monologue.