In History: When Mercy Lewis moved to Salem, she was an orphan. According to The Devil in the Shape of a Woman: Witchcraft in Colonial New England, she was a servant of Thomas Putnam and quickly became one of the top witch accusers in town. Putnam's daughter, Ann, began having fits, and so did Mercy, not long after. They believed they were victims of witchcraft, and it wasn't long before Ann and her cousin, Abigail Williams, began naming witches. Mercy later joined in, and according to court records, she accused a total of nine people.
On Salem: When we first meet Mercy Lewis (Elise Eberle), she's possessed by an evil spirit, often implicating suspected witches in Salem. Before long, though, she becomes a witch herself, thanks to Mary Sibley. Now, she's a player in the impending witch war, and she's forming her own army.