I expect DS!Yuusei can turn out differently depending on his AU, like Ryuu-twin is overly bitchy because he can't feel anything else, but dogmark!Yuu is a little more evened out in personality because he's not completely Dark Signer'd, just sharing headspace. His character is open enough to allow for all sorts of manips. Your DS!Yuu, I think, would have a sense of honor, loyalty, and actually be somewhat himself, just a bit more emo, since he's doing it to repent for his "sins", I suppose. :3