Hinokizu has the last Road Poneglyph and needs to be found to find the One Piece.
My theory is that he is Squard's former captain. His epithet is Maelstrom Spider and according to rumors Hinokizu has the ability to create maelstroms to swallow ships.
I think Squard's former captain survived the destruction of their crew too,
But I think he was captured and used for a guinea pig in Vegapunk's ancient gigantification experiment.
I think he is the zero of the Numbers like how Kuma is PX-0 of the Pacifistas. The first person the experiment was done on. This is a pattern in One Piece for groups/organizations that are number themed. Crocodile was Mr. 0 in Baroque Works. Cipher Pol has CP-0 and Reiju is 0 in Germa 66,
I think after being bought by Kaido he defected from the Beasts Pirates. I think for some reason he was able to retain his intelligence or regain it.
The manga hinted that Law knows Hinokizu and it was emphasized even more in the anime. I think it has to do with his time at Punk Hazard. His cover story was to learn about the traces of government research there.
I think Law did really do that. He was the one that revealed that the government did gigantification research to the Straw Hats. His explanation was tiny foreshadowing for the Numbers.
Shiryu believes that Hinokizu is a government man I think in a sense he will be right, I think Hinokizu was a prisoner, a ward of the state. I think him saying it might be foreshadowing too since he was the Head Jailer at Impel Down.
I think the oversize sword Squard's wield might be a memento he kept of his captain.
I think Squard's captain was a large human like Whitebeard. Whitebeard was shown side by side to Squard and it looked like the sword was right size for him.
A big hole in my theory is that an obvious clue about Hinokizu is that he is an ancient giant. I think he could have the ability to control his gigantification similar to Luffy.
