Sula is told through an all knowing (omniscient), 3rd person narrator. However, each chapter is limited to a focus on one character, with insight into that character's thoughts and perspective. Each character is given an almost equal amount of attention throughout the book so the reader is able to get equal insight into each character. In this way, there is a lack of narrator bias.
For example, chapter 1 focuses on Shadrack, chapter 2 moves to Nel, chapter 3 focuses on Eva, etc. As you read, notice the thoughts, dialogue and reactions of the focal character of each chapter. In this style, the novel comes to take on a mulit-faceted personality. Sula is less a novel about a character and her story, and more chronicle of friendship, community, and a time period in America's history.