Had to read it two and a half times, because Japanese names don't stick with me very well.
If anyone else is confused, Takei is the name of the wavy-haired blonde girl, and when the girl with straight black hair appeared in Takei's room/bed everyone just started calling her Takei and treating her like she was the wavy-haired blonde girl. At the end they see the real Takei, and realize they had been hanging out with a stranger, which is why they're shocked to see her.
Probably super obvious for everyone but me.