In The House on Mango Street, Nenny is Esperanza's younger sister, whom she initially says is too much younger for them to have anything in common. In "Boys and Girls," Esperanza writes:

Nenny is too young to be my friend. She's just my sister and that was not my fault. You don't pick your sisters, you just get them and sometimes they come like Nenny.

At this point, Esperanza views Nenny as a responsibility and a burden. She has to look after Nenny and does not particularly enjoy doing so. Nenny does not understand any of her jokes and cannot share her secrets.

However, as the narrative progresses, Esperanza comes to feel closer to Nenny and to see the similarities between them. In "Laughter," she notes that her laugh sounds like Nenny's and that they even laugh at the same things. When Esperanza observes that a house looks like houses she has seen in Mexico, Rachel and Lucy both fail to understand her. It is only Nenny who replies:

Yes, that's Mexico all right. That's what I was thinking exactly.

Esperanza and Nenny develop a strong bond, and the elder sister eventually comes to feel protective toward the younger, and to enjoy looking after her.