Kaigaku. He was always someone, who wanted to be the best in everything,and after he got turned into a demon by Kokushibo, his possession got even worse. So bad, that he after upper moon 5 and 4 died, that he became the replacement for upper moon 5

(Now in the infinity castle)

Kaugaku was training alone again, until he saw you... the new replacement for upper moon 4. He is so livid and angry that some "weak and pathetic" lower demon, got a rank higher then him

he mumbles under his breathe "Stupid... how did they become a rank higher then me, when I'm way stronger then them?!"