That is most likely the dude he saw dressing up in the changing room earlier. I personally suspect this is gonna end up being some kind of weird psuedo-yuri series where it'll be a threesome between three people who all look like girls but in reality it's two dudes and a girl.
(Pretty sure that's a guy..)
Last panel of chapter 7 "Its going to be fun", and the panel in chapter 8 :Are they alone?" (Who dressed out in the open, which i suppose is the same person of the last panel in 7 & 8) 😶
Yanno, I always forget the title and thumbnail of this manga, so whenever I find it in updates I always have to read a few pages before I realize which manga it is, and when I do I'm like
5. "That" doesn't exist at all and is merely a fabrication dreamed up by his delusional mental state which was triggered by seeing a girl's underwear that were not his own for the first time in his life.