Mary Jane is one of the three orphaned Wilks sisters; the others are Susan and Joanna. While traveling with the King and the Duke, Huck learns about the death of the Wilks sisters' father. Huck, horrified at the callousness and greed of the King and Duke, later intervenes to prevent them from swindling the sisters by pretending to be their uncles. Mary Jane shows her trusting nature by believing in the King and handing over her inheritance to her “uncle.”
When the sisters are asking Huck questions about England, where he, the Kind, and the Duke are supposedly from, it is obvious he is ignorant or lying. When Joanna accuses him, Mary Jane defends him. Huck decides then he will no longer be an accomplice in the fraud, asking himself rhetorically, “this is a girl that I’m letting that old reptile rob her of her money?” The situation grows complicated, however, as the plan to move to England advances and the girls’ slaves are sold, which prompts Huck to accuse them of stealing the inheritance, which he has actually hidden.
The next day, Mary Jane’s tears move Huck so much that he blurts out the whole story, and gets her to agree to leave town temporarily so that the swindlers cannot carry out their plan. Furious, she threatens to tar and feather them, then throw them in the river. Huck is impressed; he has never heard a girl speak this way. Mary Jane, moved by his admission and aid, says she will pray for him. Huck reflects,
Pray for me! I reckoned if she knowed me she'd take a job that was more nearer her size. But I bet she done it, just the same she was just that kind. She had the grit to pray for Judus if she took the notion there warn't no back-down to her, I judge. You may say what you want to, but in my opinion she had more sand in her than any girl I ever see; in my opinion she was just full of sand. It sounds like flattery, but it ain't no flattery. And when it comes to beauty and goodness, too she lays over them all.
By “sand” he means “spirit” or “determination.” When he and Jim are leaving town, and he looks back and sees a light in her window, “my heart swelled up sudden, like to bust,” and he realizes he will probably never again see “the best girl I ever see.”