Estraven! It's so good and it hurts so much! And it's also just exquisitely written - the way all of the folklore is both inside Estraven and Ai's story but also surrounding and commenting on their story.

And, this is a sidebar, this book's commitment to NOT unraveling all the emotional knots it tied in the character's soul felt deeply refreshing. I really do sometimes wish authors were MORE vague and LESS straightforward with their emotional impact.