Sakuya is a third-year student at Kotarou's school who looks after Chihaya as a guardian would, and despite having the same family name, is not directly related to her. While generally a sociably kind person, he is rather cold only towards Kotarou; Sakuya enjoys making fun of him.

Spoiler

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.