Who is Mildred MacDonald from Mrs. Mike the Story of Katherine Mary Flannigan and what is their importance?

Mildred MacDonald is the young girl Katherine meets and befriends at her Uncle John's ranch. Mildred is engaged to Richard Carlton, a lawyer. Mildred plays matchmaker between Sergeant Mike and Katherine by relaying messages from each to the other. Mildred is also a symbol of the impatience of youth, as she is anxious to get married to her fiancé. When Katherine and Mike leave for Hudson's Hope, Mildred adopts Juno because the trail is not a place for a civilized dog.