The unnamed narrator in Boys and Girls would be considered a round character (as opposed to a static character). Round characters tend to be complex, with compelling personalities. They have clear views about life, and the author lets us know that. These characters are not superficial or one-dimensional; in other words, they are not static in nature. Round characters also tend to be dynamic: we see changes in the temperament or behavior of these characters over time. Their worldviews may also change.
In the story, the unnamed narrator is both a round and a dynamic character. She is a round character in the sense that we have access to her thoughts, which, in turn, provide us information about her personality, worldview, perspective, and principles. In contrast, Laird is superficial in nature; we know little about him and are not privy to his personal thoughts.
During the day, the narrator is her father's dutiful helper, but, in the deep recesses of her imagination at night, she is a fearless defender of the helpless and the innocent. The narrator creates stories where she is placed in the center of the action and can demonstrate her "courage, boldness, and self-sacrifice." She tells us that she resents working in the hot kitchen with her mother and that she prefers toiling outside with her father.
Through her thoughts, we also learn that she is a little afraid of her father; he is a mysterious masculine figure to her, one who says little and only rarely demonstrates affection for her. Yet, she is fascinated by him. The narrator tries to avoid her mother whenever she can, however. She admits that she does not trust her mother, although her mother is kinder and more easily fooled than her father. Most interestingly, the narrator does not share her thoughts with either parent; she presents herself as a dutiful daughter at all times.
As the story progresses, however, we see subtle changes in the narrator's behavior and temperament (this is the dynamic part). She begins to rebel a little. When her grandmother lectures her about her unladylike behavior, the narrator refuses to submit to the older woman's authority. Instead, our protagonist continues to slam doors and to sit as she pleases.
Later, she helps Flora (the horse) escape from being killed and butchered for meat. From her thoughts, we learn that she both regrets and exults in her wayward behavior. The narrator understands, with a pang, that her father needs the horse meat for the foxes; her family is dependent on the income it receives from fox rearing. She also knows that her actions will add to her father's workload. However, she revels in the secret knowledge that, for once in her life, she acted independently of her parents and enjoyed a brief, delicious moment of freedom.
Later, however, she cries when Laird tells their father what she did. Surprisingly, her father is sympathetic and merely observes that "she's only a girl." The last line of the story is telling: "I didn't protest that, even in my heart. Maybe it was true." The narrator regrets making life difficult for her father, hence her tears. She is still her father's daughter after all. On the other hand, she also enjoyed the brief moment of personal agency she experienced.
The narrator changes over time in the sense that she begins to question her parents' worldview and to grapple with the discrepancies between her parents' expectations and her own. This is evident in her behavior and attitude. Instead of being dutiful as she has always been, she begins to rebel. She also decides that she will slam doors and sit however she pleases. Later, she lets Flora out for the sheer joy of being able to act independently from her parents. Our narrator is no longer an unquestioning child, but will she learn to make wise decisions? Munro does not tell us; she has only detailed the familiar path many of us take from adolescence to adulthood.