Mildred Morrison, also known by her family as Aunt Millie, is a family friend of Kit Kittredge's family.
While Aunt Millie is not biologically related to any of the Kittredges, she considers them family and raised Jack Kittredge when he was a child.
Aunt Millie is not a blood relative to the Kittredges; in fact, Margaret Kittredge finds it very countrified to call her "aunt" at all and calls her Miss Mildred. She raised Kit's father, Jack Kittredge, after his parents died when he was young. She was married to a man named Birch who has since passed on, and they were the only family that Jack knew.
Aunt Millie is very energetic, quirky, and lively. She is known for being forward and frank and has very clever ways to save money. Aunt Millie is very practical and can be strict, but loves Kit and all her friends very much. Kit points out that Aunt Millie is good at bringing the best out of people. Kit notes that she was the only person who has praised her for her appearance or called her pretty. She is highly educated and was the teacher in Mountain Hollow from the time her husband died until the school closed; she quotes Shakespeare quite often.