It is easy to see that you are confused because the story involves the friendship and conflicts of two women, Twyla and Roberta. But, the narrator and main character is Twyla, and since we get events through her perspective, it can be seen that she is the protagonist.
Twyla exhibits typical patterns of a protagonist-some type of trauma-which is her abandonment by her mother and time in the orphanage, and some kind of conflict-which would be the racial divide. Twyla also experiences change-that is her attitude about race. Roberta causes her to reflect on the things her mother taught her about race.