What is Celie's explanation when Harpo says Mary Agnes is different?

This section finally brings out the story of Squeak, renamed as Mary Agnes. She is powerless before the warden. In this section, the warden acts as the antagonist against the black community. If Sofia’s black family and friends are, in this instance, the protagonists of the story, trying to save Sofia from an unjust sentence, then this white warden is single-handedly the antagonist. He represents the much greater dismissal by the governing whites of the black community.

It is also significant that he is related to Squeak: even though they are family, the color of their skin is enough of a gulf between them that he can completely abuse and demean her without a second thought. When she comes home after the ordeal, she insists that Harpo call her Mary Agnes, for she both needs and deserves the affirmation of choosing her own name. The name is symbolically important. “Agnes” is a play on agnus or “lamb,” which in the Christian context symbolizes someone who sacrifices herself for someone else. Squeak has sacrificed herself, temporarily, for the sake of Sofia. After her ordeal, she summons the courage to continue on. Six months after she is abused, she regains her voice and starts to sing—surviving, fighting, and expressing herself through music.