Marsha is a member of Dojo Biden in "Scents and Sensei-bility". Sensei Tom explains to Snot that he uses anger to focus his students powers. In Marsha's case, she is angry that she is 14 and has varicose veins which will never go away.
Her design first appeared as the cashier for Booze Town Liquor in "The Magnificent Steven". It was also used as one of the people waiting behind Roger in "The One That Got Away" when he learns of his maxed out credit card.