March Harriet, also known as Harriet Pratt, is a grifter and escort who wears a rabbit themed outfit. Eventually, she was enlisted by Tweedledum and Tweedledee to serve as the March Hare for their new Wonderland Gang.
Her first appearance is in Detective Comics #841, where Batman investigates a growing new team of costumed crooks, "The Wonderland Gang". Led by the Mad Hatter and featuring Tweedledum and Tweedledee, they quickly amass high-class muscle from all over Gotham dressed as different Lewis Carroll characters, such as the Carpenter, the Walrus, the Lion, the Unicorn, and the March Hare.
Finding the crimes somewhat amiss, Batman eventually tracks the gang down to the House of Mirrors at the Gotham Amusement Park. He finds Tetch, the Mad Hatter, spinning aimlessly on the Teacups ride. Sure enough, Tetch is victim of one of his own mind-controlling chips, which Batman removes. He then goes in and confronts the gang's real leaders, Tweedledum and Tweedledee. While Batman is battling the rest of the Wonderland gang, Tweedledum and Tweedledee both escape outside only to be subdued by the Mad Hatter, who beats them both unconscious with an oar. Batman calls Jim Gordon to tell him where to pick the gang up, and to make sure his men know to take it easy with Jervis Tetch, who really hadn't done anything wrong.
Prime Earth (New 52)
Harriet Pratt was born and raised in East London where she met and fell in love with a woman named Lily Shaw. When they were adults Harriet and Lily moved to the United States, hoping to start a new life there. The couple would get by through stealing jewels and other valuable goods, this changed when the two were given jobs as escorts at a club owned by businessman William Warren. One night the couple went back stage where they discovered a new white noise machine created by Warren which would be able to control the minds of people. Harriet and Lily decided to steal the devices but were soon met by the arrival of a strange small man. This strange man was being chased by Warren's security. The man was able to escape but both girls were shot and left for dead by the security.
Harriet was saved and nursed back to health by the small man who revealed himself to be Jervis Tetch- the enemy of Batman known as the Mad Hatter. He told her that Warren had stole the devices from him and so wanted revenge on the businessman just as Harriet did. A few days later Harriet took on the mantle of March Harriet, as part of Jervis' Wonderland themed gang, and attacked Warren's complex. Warren was on a business trip however so Jervis and Harriet killed his entire staff instead and began a criminal career together.
In Other Media
Although she isn't in the game, a costume similar to hers appears on dozens of mind-controlled henchmen of the Mad Hatter.
She does however briefly appear in the Batman: Arkham Unhinged comic "Vicki in Wonderland".
March Harriet makes a cameo in the film alongside Joker's army of Batman villains. She usually hops around with her arms tucked foward when she's with the other villains. Strangely enough, Mad Hatter doesn't appear in the film.
While never appearing on screen, Harriet did appear alongside Carpenter in the comic "Harley loves Joker" which along with being set in the DCAU was written by Paul Dini.