Mary Anne is David and Dora's first servant. When she comes to them, they have references for her that say that she is indeed a paragon, just as her name says. (A paragon is a model of excellence.) But she is, in fact, a disaster: she steals from them and never keeps anything clean.
Mary Anne's disobedience totally freaks out both David and Dora because they are so young and new to family life that they don't know how to deal with a dishonest servant. This leads to one of their first fights, in which David asks Dora to toughen up and she cries and protests that she can't.