As far as I know, whoever it is, is still a complete a unknown.
Nobody actually knows, but It's almost accepted within the fandom that #6 could be Aogami Pierce, though of course thats far from official... People have come to that conclusion given that he is in no way included in any of the major events that unfold in any of the story arcs (apart from dressing up as a Gekota in Railgun S), and that Kamijou describes him as 'being incredibly strong willed and powerful'... Still, it's all just rumours.......
