First things first, everyone is damn sure that the message and the symbol got leaked by Kyoshiro after he attacked Komurasaki. He was even seen holding the paper. But it is a red herring. It is not so simple. Otherwise the traitor wont be spoken about and that too two times, once in Wano and once in Zou. If anything, Kyoshiro silenced Komurasaki by attacking her after she reveals that she is a Samurai's daughter. Also he was protecting her from the Shogun. That explains why she is alive. Hence Kyoshiro is a good guy. And a good guy with such an important role would be none less than one of the Nine Red Scabbards, i.e Denjiro by deduction.
Then for the traitor plot to come to a conclusion, the revelation must be shocking or it is of no use. And greater the trust, more impactful the revelation. Hence, the traitor must be one of the Nine red Scabbards. You can also make the case for other lessser Samurai like Shinobu but she saved the SH's from Kaido's Boro Breath. If she was the traitor, she could have just saved herself. The Tengu is also ruled out since he is living in isolation and was raising Tama. I doubt he has anything to do with this plot line. I dont think it is Yasu since one of the Nine Red Scabbards being the traitor is more impactful than some new character like Yasu going by Oda's style.
Now let me list the Nine Red Scabbards in the order of their appearance.
7. Ashura Doji/Shutenmaru
Kinemon, Kanjuro and Raizo are ruled out since we know them from prior arcs and they have been on the run in different islands. They were with Momo and did not cause him any harm
Inuarashi and Nekomamushi are also ruled out since we saw their loyalty in Zou.
Ashura Doji is also ruled out since he is a thief and refused Kaido's recruitment request.
Kawamatsu is in the prison for years and barely talked to anybody and hence ruled out.
Denjiro is also ruled out for the reasons stated in the first paragraph.
Then that leaves us with Okiku. Okiku was in Wano alone and was in speaking terms with Urashima. She may even have Raizo's Vivre Card that Jack obtained to reach Zou. She may not be bad but I think it is more like Nami and Robin's scenario.
Two more things:
- She wasn't shown bowing to Lady Toki in the flashback (typical Oda hint)
- Allowed Momo to travel to Udon
And by researching some Japanese folklore, I found some thing interesting.
"Hosokawa Katsumoto, the lord of Himeji Castle, has fallen seriously ill. Katsumoto'sheir, Tomonosuke, plans to give a set of 10 precious plates to the shōgun to ensure his succession. However, chief retainer Asayama Tetsuzan plots to take over.Tomonosuke's retainer, Funase Sampei Taketsune is engaged to marry a lady in waiting, Okiku. Tetsuzan plans to forceOkiku to help him murder Tomonosuke."
Credit to @kom5