Slight spoilers:
There’s still some mysterious person behind Ultra Rare/PSY. If we assume they are going to show up soon (or so suggested Suiko), then I don’t think the staff will be so cheap as to just pull a new character out of nowhere right at the end of the season.

In other words, Ultra Rare’s secret boss is someone we already know.
Miwa?! Well, Kourin doesn’t seem to give him any special treatment when they are alone either.
Morikawa?!! You never know, the whole Kourin-chaaaaan thing might be an act. And he does look unexpectedly important in next week’s preview… He couldn’t enter the PSY shop that one time though, hm.

So it is Shin. Yeah, never trust characters with shiny glasses! Besides, it was after they went to the beach that Aichi really started hearing voices constantly. I bet Shin did something to his cards at the end of ep34?!