The cultural differences hit hard this chapter. Not knowing someone's first name after hanging out for weeks or even months (I don't remember how long they have known each other) would be embarassing as shit to me. Like, there is no way I would even acknowledge it, I would just play dumb and find another way to get her name.
@FireCamp it's actually considered rude in japan to call someone by their first name if you aren't acquainted with them, so he likely never bothered to learn her first name. Nagatoro is her last name, and thats how she goes in school. How the hell would he have learned her first name without asking it?