Faith Leslie, originally named Mary Leslie, has a hybrid identity: she is white, but by the end of the novel her sister must admit that she is culturally Pequot. Faith's hybrid identity is an allegory for the melting pot Sedgwick envisions as the solution to cultural clashes in the colonial world.
Mary Leslie is born to Alice and Charles Leslie. Alice's father forced her to marry Charles because of his loyalty to the English crown (and, by extension, the Anglican church). Between the 17th century, when the events of the book take place, and the 19th century, when Sedgwick was writing, people increasingly saw the family as the place where children were trained to be citizens. It was important to Alice's father that she marry someone loyal to the crown not only so that he would agree politically with his son-in-law, but also so that his grandchildren would be faithful subjects just like him and his parents before him. Mary and the younger Alice (later renamed Hope) represent not only whiteness, but also the English and Anglican identity that has been passed down to them.
Mary and her sister are subsequently adopted by the Fletchers, a white Puritan family, by whom they are renamed Faith and Hope. Their renaming constitutes a baptism into Puritan culture. They now both represent a bridge across the Atlantic Ocean: loyal English subjects and also Puritans, they are living proof that this religious and political conflict does not have to permanently divide families as it once divided Mr. Fletcher and the girls' mother, the elder Alice. As a 19th-century Unitarian, Sedgwick had no real stake in the Puritan vs. Anglican debate but liked the idea of religious tolerance and coexistence.
Faith differs from Hope as a symbol of hybridity in that she is once again transplanted into another culture: after being kidnapped from Bethel by a Pequot raiding party in revenge for the Mystic massacre, Faith assimilates into Pequot culture and marries Oneco. Hope wants to bring her sister back to the colonists' world, but Magawisca helps her see in Volume 2, Chapter 15 that Faith is unalterably changed:
The suggestions of Magawisca, combining with the dictates of her own heart, produced the conclusion that this was a case where ‘God had joined together, and man might not put asunder.’
The idea that it is God's will that has joined Faith to Oneco and his people not only suggests that Hope will be unsuccessful in luring Faith away from Pequot culture, but also that the cosmos does not allow culture to be stripped away, only added. Faith readily takes on new cultures, but she can't take them off again. It is not that she has become "purely" Pequot and has lost all other identities. For instance, she has converted to Catholicism, which was introduced in North America by non-English European colonists who were also vying for the land. Whereas Magawisca feels that she will never be able to live alongside colonists in her lifetime, Faith represents the possibility that one person can have a foot in every culture that is trying to exert political power in the colonies. She is aptly named Faith because many of these cultures are fundamentally tied to religion. Ultimately, Faith/Mary stands allegorically for the future Magawisca does not think possible.
