Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.

A game as dense and mind screwy as this is sure to make a second playthrough very rich.

Fridge Brilliance

Yuki and Natsuno's first interaction in the latter's prologue has the former being very doting and poking her nose around Natsuno's activities. You could say she's acting like a... mother.
Of course Yuki wouldn't realize that the Erika Aiba she's with is an imposter. She's a new transfer student; she wouldn't be familiar with Sakura High's student body outside of individuals she was already acquainted with like Natsuno and Nenji.