What are you talking about, not-even-Willis? First off, have you even played Nier? Because 2B's moveset is incredibly faithful, and I don't understand how you can fault Project Soul for doing what they always do with guests (and indeed, as arguably is the only option for a guest): they found a way to make her work with the mechanics of the host game while showing fidelity to her original design. She's well balanced towards the upper middle tier (the best you can hope for with a guest), while bringing a distinctive role to the roster, which niche is not-quite like any previous SC character, regular or guest. And who the hell gives two shits about her face--it's a fighting game, bro, not a beauty pageant.
Meanwhile, if there was an award for "guest character in a fighting game who is most completely redundant both tonally and in terms of basic mechanics with one of the main characters already in the game" Haohmaru would win hands down even if you opened the contest up to every console generation. I mean, don't get me wrong--I'm happy that there have ended up being a lot of people who have really gelled with how he plays and who am I to say "Really that, guy? Mitsu's hillybilly cousin, complete with moonshine? Ok..." So long as a non-trivial number of people enjoy him, that's good enough justification for me for him to be in the game. But to compare his worth in terms of the overall novelty that he brings to table, when compared against 2B and Geralt?