I thought the deal with his name was that everyone knew it but he just couldn't hear them saying it until Niki broke through to him. It's been awhile so my memory of this could be faulty, or maybe that didn't apply to all characters.
Characters that knew him for a while likely knew his name and either just played along with his stupid game or were blocked out. But a lot of the girls were introduced to him as just "Sensei" and didn't learn his name till recently.
I don't want to say that anyone deserves to get cheated on, but honestly how does anyone date someone for months without learning their first name and not expect that.
When she does confront him though, I imagine Noriko is going to come up and the fact he hasn't fucked her yet has fucked so many others is going to piss her off. Noriko virginity taking while being helped by Niki?
Niki never asked Akira not to cheat on her with a harem of teenage girls, but she did specifically insist that he not fuck Noriko. So really he did exactly what she asked him to, what's there to be mad about?