I believe the real identity of Who’s Who is actually that of Shimotsuki Ushimaru, the previous daiymo of Ringo. As a recap, it’s believed by the people of Wano that Ushimaru passed away during Kaido’s onslaught on Ringo 18 years ago.
However, I don’t believe this is really the case, as Kaido has shown time and again he won’t kill strong enemies he encounters and instead opts to make them his allies, even if they resist (Killer, Drake, Kid, Luffy, etc.) Ushimaru fit the bill of strength perfectly, as he was not only the master of Onimaru, but also an exceptional swordsman himself. I don’t think Oda would just throw away a random detail like this for nothing.
It’s true that Orochi may have ordered Kaido to execute every daiymo, but Kaido has already shown to not care nor follow all of Orochi’s wishes, such as when he killed old hag Higurashi for interfering with his fight. Ushimaru still being alive would be a great way to resolve the loosely ended plot-line with Onimaru as well.
– Looking at Who’s Who, all of his facial features match Ushimaru. They both share the same light hair color, wavy texture, same square jawline, lack of facial hair despite being old, same thick lips, and both have hollow cheeks as well.
– Who’s Who is also wearing a very concealing and suspicious mask, that may just resemble the Beast Pirates’ attire at first glance, but something this mask could also heavily resemble is a bulls eyes and horns, the animal Ushimaru was most fond of, wearing all over his cloak in Oden’s flashback. He may even have another variant of the Ushi Ushi Model Zoan fruit due to this. An extinct type of Ox that comes to mind is the Aurochs.