Ma Costa is a gyptian and the mother of Billy Costa. While Mrs. Coulter may be Lyra's biological mother, Ma Costa is more of a mother figure to her. When John Faa reveals to Lyra the truth about her parents, she learns that is was Ma Costa who nursed her as a child. Lyra and Ma Costa also have an important conversation about Lyra's identity in Chapter 7, when Ma Costa tells Lyra that Lyra has "witch oil" in her soul (7.9). That may sound bad, but she means it in a good way.