The House itself plays a very important part, especially in how the narrator reacts to it. She is fully aware that she does not belong there, everything about it is described in negative terms delineating everything that it isn't versus what it is. It's by knowing where she doesn't fit that she knows to where she might fit. It is similar to the concept of light and dark. We know that darkness is the absence of light, in this case her identity exists outside of this house on mango street.