Who did you have in mind?
I'll just summarize it here, since it could get long. :p
Back then, I had the idea that
Shutenmaru was the traitor, and the reason lies in the idea that he gave up ever since he lost his comrades 10 years ago, witnessed these 20 years how Kaido was unstoppable, and believed that Kinemon's plan will fail, just like
every other rebellion; him witnessing Luffy - the guy Kinemon recruited with the minks - getting one-shot by Kaido back then - would've cemented this further.
His motive was that he wanted to
prevent any more pointless deaths as what happened with
every other rebellion in the past, and this might've got him to join the fight... which was just like Oden, who wanted to ensure no one else got killed, even to his death. But with Shutenmaru, I also had the idea that he believed the scabbards lived
far enough these 20 years.. and so wants to die fighting with honour, with all of them.
But this relies on the idea that there was
no spy 20 years ago... and this idea took a massive down-turn when Oda emphasised that the spy existed 20 years ago
yet again, via Neko and Inu's fighting. Doing it twice, thru Kaido and these two, it's basically a Chekhov's gun at this point... the spy had to exist 20 years ago..
So I had doubts about this since then, and that I might've been wrong.
But Kanjuro was a close second after him... but he would've been too obvious.