Annie "Nan" Harding is a ten-year-old girl who is determined to prove that girls can do anything boys can. When her mother died, she ran wild, so her father readily agreed to send her to Plumfield when Mrs. Jo proposed the idea. She is quite skilled when it comes to caring for the boys' minor injuries and wants to become a doctor when she grows up, deciding she doesn't want any family to "fuss over", to Daisy's shock. Nan Harding is an overly energetic, unruly tomboy. She traveled a long distance to attend a school in Plumfield. The children of that school fear her troublesome ways and attempt to give her a hard time. However, her hard-headedness teaches everyone a lesson, including herself. Even if Nan's behaviour can be quite chaotic, the kids soon learn that she has a good heart and sticks up for her friends.
Nan has no time for romance; her focus is on her career. This does not stop Tom from pursing her, but in a most unusual way. In a classic case of role reversal, Tom is the self-sacrificing one, studying medicine so that he can be near to her when he would prefer to study something else. In the opening scene of Jo's Boys, Nan was “walking briskly” ahead while Tom was “pegging on behind.” He hopes for more, but for now they are just good friends. Nan's strong will and fiercely independent streak stays strong even in the face of Tom's devotion. Tommy becomes a medical student to impress childhood sweetheart Nan, but after "accidentally" falling in love with and proposing to Dora, he joins his family business. Nan remains single, dedicated to her medical career.
“Nan was a handsome girl, with a fresh color, clear eye, quick smile, and the self-poised look young women with a purpose always have. She was simply and sensibly dressed, walked easily, and seemed full of vigor with her broad shoulder well back, arms swinging freely, and the elasticity of youth and health in every motion. The few people she met turned to look at her, as if it was a pleasant sight to see a hearty, happy girl walking country ward that lovely day …”