The first person in Book 9 to understand what Jason Asano has become is trying to kill him. Vol. 9 chooses mercy at the cost of the self it took nine books to build.
the builder
HWFWM Vol. 3: The Inviolable Self, Won Through A War of Stolen Moments | REVIEW
A close read of HWFWM Vol. 3: how Jason turns stolen moments into permanent self, refuses the star seed, and emerges Resolute against a Great Astral Being.