Sakuya Ootori
About
He is the previous Rewriter, who rewrote himself to immortality to protect Minata. As a result, he turned into a tree, until one day a young Chihaya kneeled in front of the tree, prompting him to materialize and devote himself to protecting Chihaya as an atonement for his failure. He is known as the strongest familiar in Gaia, powerful enough to destroyed Tenma's Krivoy Rog (albeit injured) in one slash.
He's the main hero of Chihaya's route.