This was explained in the "New Legends of Project Soul" artbook. Raphael brokered a deal with Inferno to allow the sword to use the style it preferred in exchange for allowing Raphael to pursue his own methods of acquiring souls, that being using politics and other people to gather the souls instead of just ravaging the countryside and collecting the souls directly. It's handwavey, but they did give an answer. It is Raphael's methods that Tira doesn't agree with, which is why she raises Pyrrha to be a savage and relentless killer, to bring back the "real" Nightmare, in a more suitable host.Sucks that they had to make Nightmare keep his greatsword. What happened to Soul Edge's ability to adapt to it's wielder? Shouldn't it be a rapier when it was controlling Raphael?
Derp. Yes, of course.You mean Soulcalibur V, right?
SoulCalibur, for better or for worse, takes its story a bit more seriously than other fighting games. Particularly SoulCalibur VI, which has thrown out the what-if scenarios and the power fantasy endings where every character gets what they want, fulfills their goals, and all that sort of thing. Now, if it happens in a story mode, it's canon, regardless of who is doing it. Which makes for a more cohesive narrative, and, if they're going to take their story so seriously, then I would prefer it this way than to how it used to be, where you didn't know for sure what was canon until the next game came out to confirm it.granted, the trailer would give you also a vague idea of what is going on but at the end of the day it would still be 2 characters being available to play that would lore-wise not be able to co-exist as 2 separate characters. All I'm saying is, so many other fighting games provide you with different timeline/lore variations of the original character, hell they've even casually thrown in Tira just to make fans shut up for a second. I'm genuinely asking would you be like 100% against the idea of Amy and Viola being available as 2 separate characters?
Tira's inclusion doesn't clash with anything, as her backstory was already written to cover back to the Evil Seed, and she was a fully-fledged assassin from the age of nine. So, this is just us seeing her origin story in-game for the first time, really. Same goes for Zasalamel, as an immortal time lord representative, his inclusion doesn't clash with anything either.
I would not be against Amy and Viola being available as 2 separate characters, no, which is why I can be on board with the Viola is a clone theory, or anything else that allows them both to exist in the narrative, which, assuming SoulCalibur VII and beyond will keep up with SoulCalibur VI's "everything is canon" methodology, is a requirement if we are to have both characters coexisting.
They can both be in the same game where the transformation occurs, if they are one in the same, but unless Viola regains her memories and is able to regain her training with a rapier, there isn't a lorewise explanation as to how they could both exist if they are the same person, so after she becomes Viola, she's stuck as Viola, and it's a one-way trip.
I believe that Amy's Soul Chronicle will be giving us some sort of an out, one way or another, that will allow the two to coexist. You can call it whatever you want, but if they did make her a clone as a result of Azwel's mad science, power to them, allowing them both to exist in all future games without having to explain it.
Edit: Added replies to mokamoka827 and Volpe.