But in the Secret Ending to BBS, Ansem the wise's eyes are orangey-yellow. Just like Master Xehanort, and Terra.
AtW had never used a corridor of darkness before. So how would he get to the LoD before the gummi ships existed? Plus, why wouldn't he go kill the Organization himself in KH2 if he's so crazy h4x strong? Why use some half-baked keyblade-wielding idiot? Plus, he probably had already researched darkness before Xehanort showed up. Read Ansem Report 1 and Secret Ansem Report 1.
The amber eyes signify association with darkness, and Ansem may have already started his studies into darkness at that time. The Ansem reports do not specifically say that he started after Xehanort arrived in RG. In fact, it might have been the spread of the Unversed that made him begin researching darkness, so he might have been studying for a long time. The first Ansem Report, which states that AtW will research darkness, is stated to take place an indefinite amount of time before Secret Ansem Report 1 (in which he has found Xehanort).
Plus, there is no way that Ansem could have had one of the black cloaks. LoD is not Castle Oblivion during the Unknown fight, so the fight occurs before Terranort becomes Ansem's apprentice. Terranort only makes the cloaks as Xemnas because he remembers them from his days as Master Xehanort. And he doesn't remember any of that until
after his experiment with Ansem discussed in the first Secret Report. It's not likely that MX gave him the cloak either, as Ansem did not recognize the name 'Xehanort' when Terranort claimed that was his name. And while you could argue that Braig could have given it to him, we have never so much as seen Braig in the same scene as one of the cloaks. As far as we know, he has no use and no knowledge of the cloaks. So Ansem could not have gotten the cloak from its maker, Xehanort, probably didn't get it from Braig (here master, have a magical cloak that protects you from corridors of darkness... what do you need it for and where did I get it? Well...), and didn't get it from the memory-less apprentice he didn't have yet. So unless you're going to argue that he just happened to invent cloaks that are identical in every way to MX's, AtW couldn't have one of the cloaks.
Plus he had never used corridors of darkness before the 6th Secret Report, he didn't have a keyblade to use, he never had a star shard, and gummi ships didn't exits yet, so there's no way he could have reached LoD from RG. And he has absolutely no reason to viciously attack an innocent bystander on a different world. He was pretty mellow most of the time we saw him in Days and KH2. He always made other people do the fighting for him... probably because he's too old.
So yeah, there's pretty much no way the Unknown is Ansem the Wise.
