John Persona 6 hypothesis
John Persona 6 uses mirrors to summon persona. like others have said, its self identity and jungian. John Persona 6 is also blonde which is a typical hair color of vampires, especially in anime and manga so its highly likely John Persona 6 persona might be dracula or some vampire variant. john persona 5 has a western based persona so its not something new.
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Vampires do not have reflections (something silver yada yada). This ties well with the fact that all persona protagonists are wild cards and the fact that John Persona 6 has no reflection in the mirror would explain his ability to hold different personas within himself (persona 6 maskot: no way, john persona 6! you dont have a reflection?! thats super weird, persona users always have reflections of who they really are! that might explain why you can hold different personas!). This also links well with the self identity theme and the "who are you" in the teaser trailer. The graves also reinforce the horror theme, monsters, and importantly the story of grando, the "first real vampire" that arised from the grave to terrorize people.
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thats it for my presentation, i came up with this after reading the posts here about possible persona summoning and possible protag persona. peace