Mr. Matsuzaki is Youko's homeroom teacher at the girl's only school. He does not appear to be named in the original Japanese edition of the novels and it only referred to as Youko's homeroom teacher.
After Youko gets in trouble for falling asleep in class and disrupting a lesson when she wakes up from a nightmare, Matsuzaki has a meeting with her to discuss her behavior. Youko lies about being tired from studying, not wanting to discuss her strange nightmares, which Matsuzaki seems to accept as a reasonable explanation. However, he warns her about her reddish hair and how it often gives people the wrong impression of her.
Shortly after the conversation, Keiki appears at the school and recognizes Youko as the next ruler of Kei. However, Youko, Matsuzaki, and the rest of the faculty in the staffroom are bewildered by Keiki's appearance. When Keiki forcefully tries to take Youko away, Matsuzaki attempts to stop him. The subsequent confusion that follows after Keiki decides to make his pact to Youko and an attack by youma following him destroys all the windows in the staffroom and incapacitates all the teachers, including Matsuzaki, allowing Keiki to escape with Youko to the roof of the school.
Youko later learns through a vision from the Suiguutou that Matsuzaki survived. In the vision, her homeroom teacher mentions how Youko was an exceptional student and did not believe that she had any explicit connection to the strange man who approached her in the school. However, he admits that while Youko was well-behaved, she did not appear to have cultivated connections with anyone and seemed to lack any sort of personality. His cutting words conclude with expressing doubt towards who Youko was, suggesting either she was pretending to be good and was secretly a delinquent or that she realized how empty her personality was and simply decided to disappear out of profound sadness.