Among Batman's more popular villains are Harley Quinn and her ally and love interest, the Joker. Another popular Batman villain is Ra's al Ghul, who is instrumental in the transformation of Bruce Wayne into Batman and is equally matched with Batman, physically and intellectually.
Catwoman is one of the greatest female villains in Batman. She is unique in that she has fought against and alongside Batman. She is one of Batman's more complicated love interests. She constantly challenges Batman's perspective and morality, because her view of life is not black and white, but various shades of grey.
The Scarecrow is an intriguing villain because his game is all about creating fear. The Scarecrow is known for inducing waking nightmares through chemical compounds. His goal is to break Batman, and he targets him because he believes Batman has a loose grip on sanity.
The Joker is Batman's arch nemesis and one of the greatest villains of the Batman universe. He is the villain most people are familiar with in pop culture references, specifically because of his maniacal laughter. The Joker is a sociopath who believes the social norms are anything but normal. Like Catwoman, he persistently challenges Batman's view of morality.