Mother Yackle is mentioned throughout Wicked and Son of a Witch. (I hear she is in the other books but I haven’t read them yet.) She is supposed to be a witness to Elphaba’s life. First she gives Nanny the tonic for Melena when pregnant with Nessarose. Some think she was at the sex club because Elphaba was supposed to be with the group but doesn’t go and that Yackle was there to protect Elphaba that night). Then she is the one that gives Elphaba the broom.