"Whats The Time, Batman?!"
- Mad Hatter to Batman
In the comics, Mad Hatter was shown as a man obsessed from a young age with Lewis Carroll's book Alice's Adventures in Wonderland, Jervis Tetch, an expert hypnotist, embraced a delusion that he was the incarnation of a character in the story, the Mad Hatter. Using his skills for mesmerism, the Mad Hatter has committed many crimes, often themed around the book that inspired him and his love of hats and headgear, going so far as to implant his hats with mind-control chips to amplify his hypnosis skills. Above all other headgear, however, he covets Batman's distinctive cowl, and will stop at nothing to acquire it.
Taken from the page at Arkham City Wiki
Jervis Tetch appeared in Streets of Gotham portrayed by Mark Strong, and he was a brilliant scientist, before he quit to fullfill his dream of one day being a famed magician under the name "The Magnificant Mad Hatter". The Mad Hatter dressed exactly like, well, who he was named after. The Hatter had many tricks mainly invovling hypnosis, and hats he scientifically created. Soon becoming obsessed with headgear, and seeking to pull a big trick... turning Batmans cowl into a hypnosis-hat! When he was up on stage one night, and a strange gas was released upon the crowd, he started seeing all of the crowd as flesh-eating zombies, which were slayed by a man dessed like a scarecrow, who promised Jervis that he would help pull off this big trick, as long as Jervis takes off Batmans mask. Jervis agreed, and was taken along with Scarecrow and Dr. Victor Fries to finally unmask the Bat.