Er, wow. Lots of different opinions...
Personally, I think thatSera is some sort of BO member, mainly because it's high time that one appeared (how many chapters has it been since the last one?) and I just have this...intuitive feeling, I suppose. Does the fact that he's wearing black count as a factor?
I chose Bourbon because it was the first thing that popped into my mind at the end of chapter 768 . Of course, I fully acknowledge the fact that it's rather unlikely that Bourbon would be so...nice, but hey, they've got amazing actors in the BO, and Tomoaki Araide seemed pretty nice (Vermouth is kind of an exception, but Dr. Araide did seem quite in character).
Of course, he could just be another character that wasn't named on the voting poll, so I added that in there.
Oh, and I retain my opinion that Sera is a guy. No real info or deductions to back it up, but it appears that Sera doesn't have a very large build, so it would be difficult to disguise "him" (let's just write this as if he was a guy) the way Vermouth did with Dr. Araide.
Seeing as Conan failed to introduce himself before Sera guessed his name, I'm assuming that either Sera has inside information on Conan (aka, he's a bad guy, a theory I support) or has known Conan in the past and is currently disguised (a theory that I acknowledge, but hope isn't true, because I want a bad guy somewhere!!!)