What is special about Mary Jane? Why is she the only person to whom Huck tells the truth?

Mary Jane explains that she is upset about the slaves being so mistreated, and Huck blurts out that they will be together again in two weeks at the most, knowing the Duke and King will abandon the town. When he realizes he has slipped, he decides to tell her everything. She becomes furious as he relates the story, and when Huck finishes, she calls the King a "brute." Mary Jane is Huck's sort of girl. She is compassionate and has her own values. Huck feels both love and empathy for her.