I'm getting a feeling that Mashiro knew and was infatuated with Nagisa back in middle school.
I might be drawing a very long bow but
could it be possible Mashiro acted sick in the first chapter to get Nagisa to go to the same high school as Mashiro?
Middle school and her acting sick is a bit of a stretch, she’s been pretty clear she doesn’t like people knowing she’s weak and her dad confirmed she’s had health issues for forever.
That being said I’m still not 100% convinced she didn’t recognize Nagisa as the girl who helped her that day even through the make over. She was really quick to focus in on her since arriving and seemed infatuated almost immediately, and were kinda missing some details about the first incident.
Nagisa said she blacked out after realizing she wasn’t gonna make it to her exam and woke back up at home, that’s a pretty big gap of information we don’t know. Mashiro might have regained consciousness enough to get a good look at her face or even her name from an ID or something.
Or I’m overthinking it, and her immediately gravitating towards nagisa and the quick explanation of the inciting incident might just be what they seem to be because of the quick and simplistic approach to the writing.
But I wouldn’t be surprised if we eventually get a flashback to Mashiros side of the inciting incident at some point