Artist Disillusionment: In-universe and at the start of the series as he no longer finds pleasure in being a musician (he outright walks out on a concert he was doing in Tokyo). Lil D and his class re-ignite his passion for it.
Cool Teacher: Admit it, you'd love to have him as your teacher.
Deal with the Devil: To get Li'l D out of his. Then he acts so bad, explaining that he had no soul in his work, and thus gets his soul back and saves both of them from the devil.
I Just Want to Be Normal: To an extent, He goes back to teaching because he got tired of life as a music superstar and almost quits when a hoard of celebrities try to teach at the school.