Who is Madeline from The Broken Cord and what is their importance?

Madeline is the third child adopted by Michael. She is ten months old when Michael is given the opportunity to take her. Michael calls Dennis with the request for a daughter and both Michael and Dennis are when Madeline becomes available to Michael, who is still a single parent at that time. Michael notes the differences between Madeline and the boys, saying that she is always "delighted" with anything and that "goody" is her first word.