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~
Yeah, this is my guess too. I kinda doubt she's being introduced as a potential love rival; I think she's the girl who was shown in chapter 24 for one panel, and if she was there to witness Nagisa princess carrying Mashiro off the stage, then there's no way she could seriously think she has a shot. XD
Nagisa mentioned in the first chapter that she cut off all contact with her friends from middle school, so this new girl has got a lot to catch up on. She's also always been pretty straightforward and assertive, so I kinda doubt she'll seriously entertain anything this girl is saying in regards to distancing herself from Mashiro.