@LemonJuice She appeared as a background character in earlier chapters, so she's always been part of the friend group. I think the author wanted to save her for when it fit for Nagatoro to be on the defensive.
Wait, he can distinguish her by her footsteps? Damn son that's good, I'm actually curious if this serious looking senpai is going to be a rival, that sounds fun.
I knew it, finally this character has been teased from the very first chapter. Senpai gets influenced by the girl closest to him. First it was this white righteous girl knight (who resembles the president) that abandon his main character and then appears nekotoro. So we can assume that senpai also had a crush on the president. I don't know Nanashi but this manga seems very well planned to be so simple. I'd love to see how it developes from this point 😆