>>281631206
The Three Greats are candidates because they are Rank 10, except Sullivan who can be one but keeps refusing it. Rank 10 is a prerequisite for Demon King, not a rank which only Demon King can have. Crowns choose Demon King among people with Rank 10, which are Three Greats.

Students thought Opera could be Rank 9, which indicates that not all Rank 9 are capable enough to be Crowns. In other word, all Crowns are at least Rank 9. In short, Rank 9 is a prerequisite to be 13 Crowns, and Rank 10 is a prerequisite to be a Demon King.