Who is Mrs. Hubbard from Murder on the Orient Express and what is their importance?

Mrs. Hubbard is described as a talkative, silly, self-centered, American tourist. In reality, she is the impassioned and grieving mother of Sonia Armstrong. Hubbard's real name is Linda Arden. Linda Arden is a famous actress, thus, she plays the role of her life, as she adds her own well written and acted summation of what she witnessed the night of the murder. In the end, Arden is relieved that justice has finally been served.