Mrs. Martin is to Mrs. Smith as Mr. Martin is to Mr. Smith. Just like her husband, this lady manages to be a blander, more generic version of her counterpart. She sides with Mrs. Smith in the great doorbell debate and agrees at first that Mary, the maid, has no right to tell a story. It makes total sense that she replaces Mrs. Smith at the end/re-beginning of the play, as they're almost indistinguishable.