Board Thread:Naruto Gaiden/@comment-3279282-20150504200737/@comment-25093611-20150504203415

i will make my guess:

the sharingan are the once which was stolen by obito, the new char is a artifical leftover from madaras experiments and therefore he is partially zetsu, and hidan gave his DNA to this being which makes it immortal :D

the hidden char is a creepy looking hidan who looks like a "monster" (scar and wounds all over his body)