In the latest chapter Kiku and Raizo reveal themselves to the prisoners.
Raizo is stated to be Fukurokuju’s biggest rival as a ninja.
Kiku puts on a mask, causing the prisoners to recognize her as Kikunojo (his epithet is the “Lingering Snow”), a man, to Luffy and Chopper’s shock.
Although he is male, Kikunojo says he is a “woman at heart” and so tends to act effeminately.
In truth, there were many things hinting at it: The clearest sign of Kikunojo’s gender is him referring to himself with sessha (拙者 or せっしゃ?), an archaic form of “I” used by male samurai.
Kiku is wearing a Hannya mask. This is the important part about this character.
The Hannya mask is a mask used in the Noh theatre to represent the soul of a female turning into a demon due to obsession and jelousy, being Lady Rokujo one of the most famous representation of said characters.
The Noh theatre was represented only by male actors in the past, with some of them playing only female roles because of their femenine features (some cases the actors only wear a mask to show the public that they are females).
So Kikunojo is just like Izo from the Whitebeard Pirates. They both are men but they look and act like women.
Probably Zoro had already figured it out! XD