User blog comment:Observer Supreme/The Three-Eyed Killer/@comment-24850639-20170613171936

Oda doesn't usually do masks for no reason at all. My guess with Killer is that it's something that will ultimately play a role in the story and/or will cause an incredible revelation.

Jack - He's not the first human-fishmen/mermen we've seen for sure, but he is the first DF USER of this genetic makeup that we've seen. He has effectively thrown a wrinkle in the assertion that all DF users can be killed by submerging them in water. I think we're all 100% sure he's coming back in Wano, so the nature of it all will probably be explained then.

Duval - Not only did it hide his uncanny resemblance to Sanji's wanted poster, but it eventually served as an even deeper connection to Sanji after we learned that he was forced to wear an Iron Mask in his youth as well

Cipher Pol - Although they had a much shorter story lifespan in Water 7, the reveal that shipyard workers, Kalifa, and Blueno were CP9 is IMO still one of the greatest plot twists in the series to date. We see now that a lot of CP0 wears masks, and although I doubt we secretly know all of them, I'm willing to bet that more than a few of them are going to turn out to be shocking reveals. Rob Lucci was probably purposely done to throw us off of this so we'd no longer think so much of the masks.

Sabo - Although he practically telegraphed this one in so many ways, he still knew that there would be absolutely no denying it once we got a look above his neck. He purposefully waited until the Mera Mera No Mi was eaten before he finally let us see his face.

Judge - Heavy speculation, and not much story value, but most people believe that his eyebrows swirl and face the same direction as Sanji's, making the Sanji the one that actually resembles him the most genetically.

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The initial speculation out there was that he was one of Sanji's brothers, but there hasn't really been any new speculation on him since this was debunked. Three-eyed race is certainly feasible, though I'm personally not all that invested in believing it without more evidence.