Ive been feeling like this for a while now, but I think it may actually be Kuma, who I think is vastly overlooked as a character up untip this point.
But I argue that he is probably THE most mysterious character in the series barring Shanks or Imu. Oda has had him undergo a mind control plot since preTS, meaning his story is very old and solidly planned because of it. In that way, he is very similar to Jinbe in how he was designed. The only "current" issue is his interaction with Luffy. Its extremely minimal compared to someone like Yamato or Carrot or Vivi or Smoker.
However, he has 3 big traits that work in his favor:
- An extremely relevant and tragic flashback. However, and to argue the other side, since DR (Kyros/Riku/Rebecca and Law) to Wano (Momo/Oden), this hasnt necessarily mattered. But I argue that Oda is really laying on the "heavy" of Kuma's own tragic last. Is it tied to God Valley? His motivations of being a King? Motivations to wiping his memories? Why do it? These are all very old plans by Oda, unlike the above, because they all started pre timeskip.
- The DF number theory is old and has been used repeatedly to argue a myriad of characters. Funny enough though, when it was first mentioned via SBS, Kuma is the one mentioned and Oda ignored it:
- Kuma, like Jinbe, has done so much for the crew vs. any other character in this series. So much so that he and Jinbe are the only characters like this at all. Jinbe saved Luffy's life 3 separate times no less (Akainu's 2 attacks towards him, and donated his blood), then acted as vanguard at WCI. Kuma saved them at Sabaody and protected their Ship for 2 years.
Kuma is lingering as a mindless machine for "some" reason. I do not know what that reason is, but we are going to find put extremely soon considering he is likely on his way to Egghead.
To add, I absolutely think he's getting a new body when he gets here. We are on an island where VP has 6 other bodies and Kuma is mass produced and Seraphim are clones lol. Its super on the nose.
Tldr: Kuma for last strawhat. Genuinely mean it.