A case could be made for more than one character, but the purest is Pearl. She didn't choose the situation into which she was born. Instead, she was outcast for someone else's actions (or rather, several other people's actions), and grew up more like a natural creature than a socialized one. She's not without violent tendencies, but even these seem like natural desires, rather than sin or corruption.
Pearl is the innocent one in this story. She is the only one who does not commit a sin. Ironically, she is ostracized by her God-fearing community because she is the product of sin; her youth and virtue are not taken into consideration whatsoever.