What is the term for a character who embodies a single quality and does not develop throughout the story?

The term for a character who embodies a single quality and does not develop throughout the story is indeed "flat character." Flat characters are typically characterized by their lack of complexity and depth; they serve specific purposes in the narrative, often representing a single idea or trait. These characters can be easily defined by their role and actions, and they often do not undergo significant internal changes or growth as the story progresses.

In contrast, a dynamic character is one who experiences significant internal change or growth, often facing challenges that transform their perspective or personality. A round character, on the other hand, possesses multiple traits and complexities, allowing them to evolve and develop over the course of the narrative. A static character remains unchanged throughout the story but may still be more complex than a flat character. Therefore, the distinct simplicity and lack of development in flat characters make them a fundamental aspect of storytelling, particularly in illustrating particular themes or moral lessons.