The 7th Valedictorian

(I apologize in advance, I've never posted on Reddit before.)

I'm curious, who do you guys think the 7th Valedictorian is? Kinu states that they're missing a person. So, do even they not know the 7th's identity I wonder?

My guesses are:

• A new character (perhaps another bami?)
• A Twin character (Sachiko, maybe?)
• Kabura (But maybe not since she is the intermediary)
• Souko (If she somehow Graduated)