Considering the way he portray her, I though of Naka.
I think Sendai can beat Naka-chan, even if the latter is bloodthirsty. Jintsuu could definitely beat Sendai. And except for the star of the book, there's nobody else who can challenge her. Shigure can't. Ditto Yuudachi.
I had her down with my first glance.The first is...And if I cannot measure it, then if it came to blows, I have no idea what would happen.Eh? That person?Ah, let's just keep that one a secret."I have no idea what will happen"The other one... well...I couldn't even fathom her depths.And since I cannot see them, I can't measure her ability.Such a nebulous individual - I'd really rather not fight with someone like that if at all possible, see?