The Joker

The Joker is a serial killer, a dangerous madman who dresses like a clown and commits violent crimes. He is often recognized as Batman's greatest enemy, living in and terrorizing Gotham City, although he spends most of his time in Arkham Asylum. In addition to being a deadly and unpredictable physical combatant, he is a brilliant and ruthless criminal mastermind. His crimes require no motivation other than his sadistic desire to show people the meaninglessness of life through pain and death, and the narcissism to see the world remade in his own image. He is constantly adapting his personality and his psychosis to respond to the world around him, sometimes a harmless trickster and at others a brutal mass-murderer. Batman is his only worthy opponent, and although he has passed up multiple opportunities to murder the dark knight, he has no such problems with killing Robin or crippling Batgirl. Although his origins remain unclear and shrouded in mystery, it is also known that prior to the chemical accident that scarred his mind and body he was the small-time criminal Red Hood.

Mission 1 Part 1
In Mission 1 Batman and Green Arrow must defeat Killer Croc, Joker and Cluemaster. Then Joker escapes and teams up with Hugo Strange, Solomon Grundy and Black Mask. They then attempt to kill Catwoman and Connor Hawke but they are saved by Nightwing and Green Arrow.

Mission 1 Part 2
Joker teams up with Merlyn and Captain Boomerang to kill Robin. They ultimately fail and are taken to Blackgate Prison. Joker breaks out and kills Harvey Bullock.

Mission 2 Part 1
Joker goes on a killing spree as he kills Harvey Bullock, Lucius Fox, Renee Montoya and Alfred Pennyworth. Batman avenges their death by "accidentalyy" killing him.