The principal character, Jo, 15 years old at the beginning of the book, is a strong and willful young woman, struggling to subdue her fiery temper and stubborn personality.

Second oldest of the four sisters, Jo is boyish, the smartest and most creative one in the family; her father has referred to her as his "son Jo," and her best friend and neighbor, Theodore "Laurie" Laurence, sometimes calls her "my dear fellow," while she alone calls him Teddy. Jo has a "hot" temper that often leads her into trouble. With the help of her own misguided sense of humor, her sister Beth, and her mother, she works on controlling it. It has been said that much of Louisa May Alcott shows through in these characteristics of Jo. In her essay, "Recollections of My Childhood", Alcott refers to herself as a tomboy who enjoyed boys' activities like running foot-races and climbing trees.