why does Housseini include Shanzai's character

Shanzai is included because she's the children's teacher. As the "daughter of a poor peasant," she went uncovered and didn't force the other girls to do so either. In fact, she expressly forbid it. Shanzai believed and taught that, "women and men were equal in every way." Her teachings and beliefs are considered radical within society, but she is non-conforming in every way. Nothing she does adheres to the teachings of Islam, and her actions are indeed both dangerous and uncommon. She teaches the girls that the world is full of possibilities if they are brave enough to reach for the things they want.