The two characters are drawn to each other for a variety of reasons. I think most importantly, Jeffrey (Maniac) and Amanda are not preoccupied with race. Amanda and Maniac are not color blind. They both can see that people have different skin colors, but neither thinks that the color of a person's skin matters.
Another reason that Maniac and Amanda are drawn to each other is their mutual love of learning. Maniac loves learning and loves reading. So does Amanda. She even carries around her books in a suitcase. That's devotion.
I think another reason that they get along so well is that Maniac is looking for a stable home. He's looking for structure and rules as well as love, and if he doesn't find it, he just runs away again—literally. Amanda and her family give him the love and structure he's seeking. She is kind, generous, and friendly, but she is also authoritative (bossy) and demanding. She's basically exactly what Maniac is looking for in a family member, and he acts exactly how she wishes Hester and Lester would.