I’m calling it now. The head honcho that underboss (koinami?) answers to is the admin. The one the underboss said was a real sadist. Her scary face wasn’t cuz she’s a stuco member, but cuz she’s the gang leader.
The final caption does indeed describe the admin, Yagura Riko, and this is the word it used specifically.
Literally translated "black curtain", it's a term that I believe originates in kabuki. A wordier definition would be "Those who do not appear on the surface, who instruct and make a measure. behind the political world."
While "mastermind"
is a valid translation, I'm being a little reserved in using something that would imply little Riko has a leadership role in whatever criminal organisation is shaping up. Koinami refers to a "queen" she is working for in a previous chapter, I translated this as "her majesty" for the sake of clarity, but when we've seen this character in silhouette they clearly have braids, which Riko does not appear to. Additionally, Niina had no special reaction to Riko until the end, despite worrying about her failure becoming troublesome for Her Majesty.
A more direct translation that doesn't imply leadership necessarily is "fixer", but I shied away from using that as well because it's still entirely possible that Riko could do a hairflip to reveal her braids, and suddenly it turns out she's been Her Majesty the whole time. I've already had to pull off some fancy footwork with the whole detective arc in order to make previous translations and the big turnabout scene feel congruent, I didn't want to get myself into a situation where I had wrongly implied Riko was merely another lackey.
Hence, "inside man". we'd suspected there was someone inside the student council and Itsumi's proof left a clear opening for it, Niina got the code from someone else, and here at last we see who was inside the student council the whole time.