Joker

Joker is an insane criminal and an enemy of Batman, residing in Gotham City

Early life
Joker became known to Gotham City and its vigilante, Batman, after numerous criminals acts. He perceived as insane and fought Batman on multiple occasions.

Starro Siege (2020)
Joker was mentioned by Alfred Pennyworth during Starro the Conqueror's attack on Earth.

Personality
Joker is apparently deranged and declared insane, likely making him unpredictable, dangerous and fearsome in nature.

Physical appearance
Joker is described and compared to a clown, meaning that his appearance is comparable to one. Whether or not he wears make-up to look like one is unknown.

Abilities

 * Intimidation: Joker apparently has a fearsome nature due to his insanity.

Weaknesses

 * Mental illness: Due to being insane, Joker is unable to think like any regular human being. Although this makes him unpredictable, his mental illness can also be a hindrance.

Season 1

 * Starro Attacks! (mentioned)

Behind the scenes

 * In the comics, the Joker is Batman's most famous and recurring enemy, being described as a crazed serial killer, psychopath and lunatic. Sporting the look of a clown after falling into a vat of chemicals, Joker debuted in Batman #1 in 1940.