@jangankangkung I think calling girls -kun is pretty uncommon, but not like impossible or unprecedented. Maybe it's meant to imply he doesn't see her as a girl (Tomo-chan flashbacks ahoy).
@jangankangkung@JunkKing
She is one of those cool girls that popular with other girls so thats why he called him kyun there and why she was angry at his remark. Basically its a sarcasm. I think he normally just call her okamoto without suffix. Same with how he called the MC Sanada.