Who is Mary Nell (M.N.) Fickett from Lisa, Bright and Dark and what is their importance?

Mary Nell (M.N.) Fickett is friends with Betsy Goodman, Lisa Shilling and Elizabeth Frazer. She is a minister's daughter, and she used to live in California. Betsey is described as intelligent and "fantastically popular with boys". M.N. provides Betsy with advice about boys and clothes. She is a liberal thinker, an excellent researcher, and she loves to be in control.