Who is Karin Baum from Floating in My Mother's Palm: A Novel and what is their importance?

Karin Baum was one of Hanna's childhood friends. An incident with Karin's grandfather, however, left Hanna unable to play at Karin's house, causing a rift in their relationship. In the seventh grade Karin became pregnant with her grandfather's baby. While Karin was in the home for unwed mothers, Hanna often visited her, and they became good friends again.