15.
Their mother is heading outside, looking for them of course. The apartment complex is quieter around this hour, the two hour window between work ending and hapless employees begging off drinking with their boss and various other supervisors. Nagisa knows that if their mother turns around at the wrong moment, she'll see. And she doesn't like her mother, but surely the woman doesn't deserve to die for it.
The assailant's hand twitches, cuts in, and Nagisa bleeds.
It's not an unusual feeling. It's not even a particularly bad feeling. It's too shallow to be anything but a super annoying itch and it's a little breezier at their neck but it's tolerable because Nagisa knows it'll heal fast with a couple quick hunts (the nearby elite middle school is a really good place for that, and they don't know why but their mom wants them to go there so they'll probably find out.) and some cubes and ten minutes.
"Nagisa, what are you doing outside you have class in the morning?"
"There's a podcast on my phone about how to separate the events between historical eras," Nagisa tells her, and it's not a lie, there is, they just listened to it yesterday. "I came out here so I didn't disrupt you making dinner. I'll be in soon."
She pauses at the door. Then she harrumphs. "Don't forget to do some actual book work."
"I won't, mom."
The other girl's sweat drips and Nagisa twitches.
The knife digs in but she flees, taking it with her.
Nagisa makes a face. What a jerk, stealing people's stuff. If it wasn't for the fact that it was magic, they'd be down a weapon.
Still, they clean up and head inside. They'll probably never see her again anyway.
(Nagisa is dead wrong of course, but in their defense, they're not Akemi Homura, and she's got confirmation bias on her side.)