What is revealed about Mercedes in the dialogue in paragraphs 8 through 15 of chapter 5?

Mercedes has compassion for the dogs and doesn't want to see them whipped.

But Mercedes interfered, crying, “Oh, Hal, you mustn’t,” as she caught hold of the whip and wrenched it from him. “The poor dears! Now you must promise you won’t be harsh with them for the rest of the trip, or I won’t go a step.”