Akainu may be the son of Rocks D Xebec. His age is 55, so completely possible.

First and foremost, I would go back to when Whitebeard first saw Akainu and immediately called him magma boy. There seemed to be some familiarity considering how he never addressed Aokiji or Kizaru that way. Both of them had a grin when they attacked each other from long range… whereas in all other cases Whitebeard was just fighting without any sort of specific expression.

Then comes the most important fact. We have seen his childhood pic. He was in tattered clothes and he had a dagger with him looking as if he had been in a serious fight. We know that Rocks Pirates fought to death in board their own ship…would make sense if Xebec since the very beginning of his piracy career made his son go through the ordeal so that he grows strong.

Akainu says he will never forgive pirates and will kill all of them. But he literally publicly announces that Whitebeard and all his commanders can run and he won’t chase them as long as he can kill Ace and Luffy.

We know for a fact just how much of a legend Whitebeard is, so if Akainu’s goal is really just to exterminate criminals Whitebeard should have been on the top of his list. However, he shows a weird obsession towards the Monkey D and Gol D bloodlines. Right now, instead of exterminating all pirates, his main goal is just killing Luffy.

Also, Akainu killing children based on their birth makes sense. We can definitely say Xebec was trying something similar against the Celestial Dragons in God Valley.

Then comes the fact that Akainu knew for certain about the existence of the Rocks Pirates, and he also knew just how strong they were and possibly also about their line-up. He was just 17 back then…so why does he know about them so confidently? Unlike Hina who said that she specifically did her research regarding this, the same cannot be said about Akainu.

Then comes a slight similarity: Xebec was evil, Akainu is evil. Also, we know Garp and Roger took down Xebec. Luffy is Garp by blood and Roger by Will. Blackbeard probably has Xebec’s Will, so if Akainu has Xebec’s blood, it will be extremely interesting because we know Luffy is taking down both of them.

The history will repeat and basically Garp and Roger will again take down Xebec.

Theory by Ryuzaki (https://www.reddit.com/user/AffectionateWheel761)