Oooooo an interloper! Okay, prediction time:
I'm guessinggggg, she's someone from the same middle school as Nagisa who admired her for being the serious model student and stuff. But now Nagisa looks all flashy, so the First year thinks it's Mashiro's bad influence leading Nagisa astray and tries to "pull her back to the right path". Or smth along those lines~
Not even just her looking flashy, but also not making it into her chosen much higher ranked school. Amusingly enough, technically Mashiro actually is the cause for this, but certainly not in the way new girl thinks.