Rosa is Liesel's foster mother. When we first meet her, we don't necessarily trust her with Liesel. We're nervous. And let's face it, Rosa is down-right abusive with her wooden spoons and her constant scolding and criticism.
When Max arrives on the scene, we, and Liesel, see a very different side of Rosa. She's always super-kind to Max and clearly believes that hiding him is the right thing to do. She shares her food, her love, and everything she has. Helping to care for Max and seeing his suffering produce an all-around change in Rosa, especially in terms of her relationship with Liesel.
In the end, Rosa stops the abuse, though not the endearingly foul language that she, Hans, Liesel, and Rudy all use. Although she will never be to Liesel what Hans is, she becomes a beloved figure. Rosa goes from wicked foster mother to comforter and even role model.