Sunset replaces canon Twilight in this AU. Where canon Twi’s problem was her antisocial tendencies and dedication to her studies, Sunset’s problem is her arrogance and inability to understand when she’s going too far. If she can do something, she just does it, with not a single thought about possible consequences. Despite being arrogant, Sunset isn’t a bragger. The mane six are unaware that she’s friends with a literal princess or that they regularly exchange letters, because why on earth would she tell them about a random pony she knows that they don’t??? At most she offhandedly mentions her “friend Twi from Canterlot”.