The Four Yonko
These are the mightiest pirates in the sea, able to directly oppose the Marines. Each one has a quarter of the New World. The strongest is a man known only as Scratch. He's reputed to be a complete monster, levelling entire islands just because he can.
Beneath him are three Yonko, two of whom are presently in open conflict. Zetsuko, often called the Mad Maiden of Blades, is renowned as the world's greatest swordsperson. Unfortunately, she's also extremely unstable. She has been known to be very kind natured, and has a soft spot for orphans. However, she has anyone who comes to one of her islands who doesn't meet with her executed by crucifixion.
Her rival is Commodore Percival. He was reputedly an Admiral with the Marines, though the Marines themselves deny it. He travels the seas in an extremely powerful ship, wielding a huge arsenal of terrifying weapons.
The fourth Yonko is Umaru. He claims to be the son of Gold Roger, and the rightful heir to One Piece. This is backed up by the fect that the sea seems to respond to his beck and call: Storms subside for him, then instantly come back with ten times the strength to tear his enemies apart.
The Marines are led by Commander-in-Chief Shikibu, a decorated soldier who sells his image on clothing and action figures. Beneath him are three Admirals: Shigure, Wado, and Reitetsu. Each one has a rare devil fruit, though the specific ones are top secret.
The Shichibukai are pirates that have agreed to serve the Marines. They can do as they please, so long as they come to battle when the World Government asks. There are presently seven of them. In no particular order, they are:
Yuji, the Gambler. A drifter, he spends most of his time wandering the blues. True to his ephitet, he gambles most of the time, although there is a lot of drinking involved as well. However, despite his laid-back nature, he's quite worthy of his title as a Shichibukai, able to summon an entire crew in seconds.
The Masked Man is an enigma. He travels alone. He needs no crew, as he is one in and of himself. His immense size, god-like strength, and colossal axe allow him to tear enemy ships in half, all while bullets flatten themselves and swords break against his impenetrable skin. The mere sight of the mask welded to his face has made grown men faint.
Jacob Black is regarded as the prince of the underworld. It's widel said that has had a hand in every crime ever committed. This is probably and exaggeration, but not much of one: His criminal network spans the entire world, and he is behind almost the entire slave trade. He is also one of the richest men who is not a World Noble.
Pierrette is one of the world's smartest people, having made many scientific advancements. Unfortunately, most of these advancements involve kidnapping children to turn them into supersoldiers, or obliterating an entire island to test out a home-made plague. The government gave Pierrette her Shichibukai status to make her give her inventions to them instead of the Yonko.
Orpheon, like The Masked Man, travels alone. However, unlike the Masked Man, he does not fight alone. He controls people, turning them against each other while he sits in the shadows and watches the madness unfold.
Sigurd Jack is one of the more benevolent Shichibukai. He joined because the World Government agreed to give him and his crew as much alcohol as they need. As soon as they did, they had to raise taxes. Sigurd, along with being a giant, has a strange effect on people, making them become drunk seemingly on nothing more than the atmosphere he prjects with his infectious laugh. He takes great pride in being the few people who regularly defeat their opponents by inviting them out for a drink.
Anubis was offered his position after he came back from the dead. The marines sliced his head off, and reportedly, his body picked it up, put it back on, and then gave the order for his crew, who had surrounded the base while the marines were busy trying to stop him from escaping, to kill every Marine at the base. He now travels the sea in a pyramid-shaped ship, collecting every speck of gold he can find.