Gold Roger the King of the Pirates and Dragon the Revolutionary. To think that the two of you were adopted brothers all these years. What a victory it would be to end both bloodlines. Everyone can escape, the two of you are the only targets I'm after.
~ Akainu swearing to kill Luffy and Ace
Akainu (real name: Sakazuki) is one of the main antagonists of the anime/manga, One Piece. He is one the three Marine Admirals and one of the most powerful fighters the Marines have to offer. He is the current Fleet Admiral as Sengoku's successor. He ate the Magu Magu no Mi (Magma-Magma Fruit) making him become living magma. He is cruel, heartless and believes in the Marines motto of "Absolute Justice", which is to punish criminals at any cost. He will kill anyone that he thinks are evil or are an obstacle of justice. He will also not hesitate to kill his own comrades who run away from the battle. He will kill them no matter how they beg for mercy.
His debut in the series features him immediately crossing the Moral Event Horizon by destroying a ship filled with innocent refugees of Ohara on the minuscule possibility that a criminal might have been on board, an action that horrified even his comrades. Since then, he rose through the ranks of the Marines and eventually became one of the Admirals, alongside Aokiji (who detested him for his cruelty and fanaticism), and Kizaru.
He is the one who killed Monkey D. Luffy's brother, Portgas D. Ace. His magma overpowered Ace's fire power, and punched him straight through the chest as Ace was trying to protect Luffy. He even tried to kill Coby when he said further fighting would be pointless, but luckily, Shanks saved him just in time. Through this horrible action, Akainu accomplished what no other villain ever could: completely breaking Luffy's spirit.
Within the two year time-skip, Akainu officially became the new Fleet Admiral officially placing him as the commander of all the World Government's forces, which was later revealed to the Straw Hats by Jinbe at Fishman Island. According to Jinbe, since Akainu's commandeering of the Marines, the marines have become noticeably cruel under his ideologies of Absolute Justice.
Fleet Admiral Sakazuki became furious at Admiral Fujitora for revealing the true news of Dressrosa incident without even telling him. He asked why he did not report to him before taking action. Fujitora replied if he report to him, he would have cover-up the incident. He suggests that cover-ups were needed to save the face of the World Government. Fujitora doesn't want to become a hero for something that he did not do. He argued to the Admiral about the credibility and dignity of the Marine Headquarter. Sakazuki banned Fujitora from every single Marine bases unless if he can capture both Luffy and Law.
So that's him. Not only is he the grandson of Garp the Hero, he's also the son of the Revolutionary Dragon. We'll have to eliminate him as well.
~ Akainu about Luffy
He got himself stabbed because one of those foolish sons of his believed every word I said, and now he'll die to protect that same fool. Truly an empty existence.
~ Akainu to Ace about Whitebeard forgiving Squard
If man does not live righteously, he does not deserve to live at all!
~ Akainu to Ace
I'm not gonna let Law and Straw Hat do whatever they want!
We have to be thorough! If even one of those scholars made it onto that ship, all of the sacrifices we've made would be for nothing! We must eliminate the very possibility of this "evil" existing!!
~ Sakazuki's view on destroying the ship of survivors during the Buster Call at Ohara