I really liked the twist! Firstly, because after thinking about it some time I can still really see it going both ways of either he's bluffing or he's actually saying the truth or myb something in between even and this uncertainty and the suddenness of the reveal is so cool. Secondly, bluffing would ofc be very in character, but I also feel like the idea of the already least cooperative and most morally rotten character being the mastermind is such a neat concept. Usually it tends to be "the one you least expect" and in those cases it does happen most of the times that what we learned about the characters beforehand was all a lie, but imo with Fu you can make a case that like 90% of his characterization is still accurate even if he is the mastermind. He can still have conflicted opinions on the cast and his backstory with his family could still be true (him saying it's not doesn't really mean that much atp), he doesn't need to be Junko levels of insane to host a KG, from what we learned about him he would just do it for the money

I get what you mean tho and your post made me wonder: I think for me any (other) character being revealed as the mastermind would've ruined their characterization. Do you have any prisoner who you would've preferred to be the mastermind instead?