Clark is both the protagonist and the narrator of "A Wagner Matinee" by Willa Cather. The only other significant character in the story is his Aunt Georgiana, the woman who raised Clark as her own son.
The protagonist of the story is usually the main character who undergoes change, and in this story that is Clark. It is true that his aunt also undergoes a change, but hers is more from just living life than from any kind of epiphany, which is what Clark experiences.
When Georgiana was a young woman, she loved the arts and culture of every kind and was particularly gifted in music; however, she chose a different life and was forced to work hard to care for her family on the plains of Nebraska. Despite that, she always helped young Clark gain an appreciation for such things. She did give him this warning, though:
“Don’t love it so well, Clark, or it may be taken from you.”
Clark now lives in Boston and learns that his aunt is coming for a visit--the first time she has been off the farm in thirty years. To repay her for caring for him and to thank her for instilling in him a love of the arts, he plans to take her to see a Wagner symphony matinee.
When she arrives, he is embarrassed at her frumpy and old appearance; he is even more disturbed by her apparent detachment from anything but life on the farm. He almost cancels their outing, afraid it may be more than she can bear to hear fine music again.
Aunt Georgiana is moved to tears by the performance and does not want to leave; nothing has changed for her. Clark, however, realizes that what his aunt appears to be is only part of who she is. Inside, her passion for art and beauty is still strong, despite the hard life that tried to leech it from her. Because he is the one who undergoes a change in thinking, Clark is the protagonist of this story.