– Even looking at Who’s Who’s sword, it definitely looks like a meito that comes from Wano, with the flower/cherry blossom design that is. It’s also known that the samurai in Ringo wield the same blade from birth, until they die. Thus, this could just be the sword Ushimaru always used, especially since in the flashback, Ushimaru was shown wearing the exact same ‘scattery’ (for lack of a better word) flower design all over his kimono.
– The guard of his blade even has the same bumpy curves as other Wano swords like Enma, Ame no Habakiri and Kiku’s sword, for example. All of this would explain why his sword has such a peaceful design, despite him seeming to be just another subordinate of Kaido. Also, considering tattoos don’t seem foreign to Who’s-Who, I wonder if he’d have a crescent moon tattoo on his ankle if he lifted his pants up as well.
Of course, with a name like “Who’s Who” it makes a lot of sense to begin with for him to actually be someone with an underlying identity, who has been biding his time patiently until this fated day, just like Denjiro and Yasuie. This would also fit nicely into the huge theme present in Wano so far regarding secret identities!
*Theory by TKodyssey