It is evident that Miss Celia needs Minnie. She realizes this. It becomes clear that Minnie is Miss Celia's only steady stream of human contact that is not shrouded by facade and deception. Miss Celia was born in poverty and feels out of her element. She can identify with Minnie, who knows what it is like to be on the outside of a social world. Minnie recognizes all of this. She may complain about Miss Celia, but she does show her how to cook and bake with more than mere professional responsibility. When Miss Celia suffers the miscarriage, she helps her through the ordeal. At the same time, when Miss Celia recognizes that she is an outcast and wants to go back home, Minnie encourages her. She tells her that to leave results in acquiescing to Hilly and she wins.
In these examples, it becomes evident that Minnie does care for Miss Celia. While she might complain about her, Minnie realizes that Miss Celia took a chance on her when no one else would. At the same time, it becomes clear that both of them rely on one another as they struggle through their differently challenged predicaments.