Comparatively little takes place in this story, yet it really struck me as being powerful. It seemed to me that here was a character who truly embodied the antithesis of the Mary Sue--when an author creates a character that she uses to live out fantasies of her own life, without giving her any truly meaningful challenges. It took a while of musing on why exactly I was so affected by this story.
It comes down to something said in this talk: David Brooks on "Should you live for your resume or your eulogy"?
He talks about two selves we all have--one that focuses on building one's strengths, and craves success, and one that focuses on battling one's weaknesses, and on charity, love, and redemption. Only the latter creates depth of character. He quotes Reinhold Niebuhr--"Nothing that is worth doing can be achieved in our lifetime."
Mary Sue stories (Twilight is a good example, in which the only weakness focused on is Bella's physical clumsiness) focus on characters' strengths; the backdrop of the story and the plot are only meant to showcase those strengths. Tehanu, on the other hand, focuses on testing the characters' weakness--and the story is about the building of depth of character. Not only does Tenar care only about charity, love, and redemption, but she eschews recognition for her past achievements.
We can admire Mary Sue characters, and fantasize about being them, but the fiction is ultimately escapist. There is a place for every kind of fiction, of course. But when I read something that somehow keeps me in reality even as I'm indulging in fantasy--something in which I truly feel like I'm relating, that my brain synapses are firing in new patterns as I align myself to a character's search for identity, purpose, ways to embody humanity in their being and actions--there is something truly special about the experience.
It's hard for fiction, especially in the fantasy genre, to show us what we can do in our lives. We don't have much cause to battle evil in the form of paranormal creatures or magicians. But fantasy can, if done right, show us what we can be, and as it taps into the mythological archetypes that exist in our subconscious, maybe it can do so even more than other genres.