Susan Sedgewick, "Elizabeth Freeman (Mum Bett)" watercolor on ivory, 1811.Freeman, Elizabeth (Mum Bett):

(1742-1829) Mum Bett was born into slavery and was enslaved until her 40s. At this time, her master attempted to hit her sister but Bett got in the way and was struck instead. She sued the state of Massachusetts for violated her state constitutional rights, saying that she deserved freedom based on the law. She won the case and became one of the main reasons why Massachusetts banned slavery.
http://www.pbs.org/wgbh/aia/part2/2p39.html

Source: Horton, James Oliver, and Lois E. Horton. Slavery and the Making of America. New York: Oxford, 2006. 66.