What term describes the main character in a story, particularly one who undergoes significant change?

The term that describes the main character in a story, especially one who experiences significant change, is the protagonist. The protagonist is the central figure around whom the narrative revolves and often faces challenges or conflicts that lead to character development. This character's journey is usually marked by growth, transformation, or a deeper understanding of themselves or their environment.

In contrast, an antagonist is typically a character or force that opposes the protagonist, often creating conflict. A foil serves to highlight particular traits of the protagonist by contrasting with them, but they do not experience the same depth of change. Supporting characters play roles that assist or complement the protagonist but are not the primary focus of the story's arc.