I've mentioned it before but if the mastermind is just alternate Yotsuha I'm dropping this manga. It's a purely unimaginative and boring plot twist, it would be more of a twist if it was someone else at this point, since the audience is already being led to believe an alternate her is the one. I can think of very few situations where that twist can be made interesting.
I'm guessing that it's a character that hasn't been introduced yet, if the author has any creativity it is at least, and the twist might be that it's someone who has it out not for Uichi but for Yotsuha. It's gotta be someone who already knows Yotsuha is doing what she's doing, so maybe someone else with a watch or someone who ended up with hers somehow. They're trying to get to get through Uichi, maybe trying to make her break.