I don't think I have to walk around the block when I say that it's pretty obvious who's going to be the least voted.
The possibile candidates are two, but I can see a pink haired girl to increase in popularity and appreciation after the release of Last Round.
For another certain tragic weapon clone the situation is more unlikely but who knows, maybe with the right balancing fixes and with some additions/changes from Kasumi's moveset (that I'm sure will appear in DoA6, as natural evolution of the character as happen for everyone) the things could change.
Look to Leon, when DoA2 was released I'm pretty sure that he ended to be hated and disliked by everyone for replacing Bayman as clone of him, and after the return of the Russian fighter I believe that the reaction of the people was "Bye, bye Leon, now I return to my real main!" And look just now, currently he is usually considered a fan favourite from the general community of DoA that is not interested only to the sexy girls. So never say never again
I find all of new characters to be appealing, some more so than others.
I couldn't think to find someone thinking the same, this is exactly how I feel about the additions. I can't say that there are characters that I dislike or that I think ended to appear wrong, I find all of them interesting even if some more than others.
Actually the same process didn't happen to me with a few of the DoA3 and DOA4 additions during their release, even if with the time (and especially through the the DoA5 restyling for design and personalities) I ended to like them too.
Nyotengu is also beautiful to look at while retaining the fighting style of another boss character... there is also something about her that I find comical (or "unintentionally" funny). Nyotengu is a recurring character for me as well.
Nyotengu appear even to me clumsy and in part silly in certain taunts or animations, I don't believe that this is unintentional because Bankotsubo was in the same way (the only boss of the DoA series to not be seen completely in a serious way, even if I have the impression that even Raidou, now that is associated to Honoka, could be showned in this prospective from time to time).