What is this smile and why does it make me so angry ?
You expected be happy that Slutueno got dismissed, but that made Nishimiya uncomfortable and somehow guilty to the point that she's almost crying while forcing a smile in that panel.
Because Ueno you're jealous.
She doesn't want compassion. Or, as Ueno nicely put it, "playing friends", putting on a facade. Remember how seemingly happy she was to trade punches with him, when they were kids?
It was this.
Those rare moments when, for better or worse, the other drops all pretences and shows their real face.
Now, Ueno had just a very same moment when she got that ride home.
Nishimiya probably feels uncertain now, wondering if what he does for her is the real him, or just a pretend-play. Which is justified, because we know he doesn't really understand the "why" himself.
I want somebody to punches someone in this manga.
do u have a heart nigga