The guardian who trained Zero to fight, and who told him plainly that raw power was not enough.
Who he is
Precise, dignified, a scarlet scarf that "seemed to move with its own purpose" (ch. 1). He fights with manifested energy and bone attacks.
His assessment of Zero on meeting him was one sentence: "Your combat experience is non-existent. We'll need to address that immediately."
What he taught
He turned Zero's raw manipulation of reality into technique. His warning is the one that matters, and the Prologue proves it twice over:
"Your power is considerable. But power without precision is as dangerous to allies as it is to enemies." (ch. 1)