Dr. Vegapunk. Kuma talked about him when they faced the real deal on Thriller Bark.
Spoilers lie ahead.
*takes deep breath*
The Strawhats go to the Mangrove Forest to get their ship coated so they can travel to Merman Island. The 11 rookie "supernovas" are in the Mangrove Forest because all the routes to One Piece converge here. The Mangroves are right next to Mariejoa, which is the Marine's HQ and where the Tenryuubito live. The Tenryuubito are the descendants of the people who set up the World Government and they are treated like Gods. Hitting a Tenryuubito causes Marine Admirals to be dispatched to the Mangroves. Luffy and his crew hit not one, but three of them while trying to save their mermaid friend from the Human Auction. Marines are dispatched, including Admiral Kizaru and several "Pacifista" cyborgs that look exactly like Bartholomew Kuma and have the laser abilities of the Marine Admiral Kizaru. The Strawhats escape the Auction house with the help of "Dark King" Silver's Rayleigh the former first mate of Gold Roger himself, the Heart Pirates led by the coolest pirate alive "The Surgeon of Death" Trafalgar Law, and the Kid Pirates led by the pirate with the lamest nickname ever Eustass "Captain" Kid. They talk to Rayleigh who agrees to coat their ship so they can go to Merman Island. The Strawhats are instructed to lay low until he finishes. So naturally they go wandering around the Mangroves. They run into a Pacifista and take it out. Elsewhere Kid and Law run into another one, the Don and Bonny make a hasty retreat through some more conventional Marines, and Urouge, Hawkins, and Drake face off against Kizaru and a Pacifista while Apoo watches from afar. Eventually Apoo lends a helping hand and uses his music abilities to (probably temporarily) take down Kizaru somehow. Meanwhile, the Strawhats are the only people on the Mangroves not to know that the Marines have scheduled the execution of Luffy's brother Firefist Ace. The Marines know this will bring the infamous and super powerful Whitebeard to Mariejoa to save his man. They expect a war and therefore have gathered the remaining members of the Royal Shichibukai and presumably as many Admirals as they could get their hands on to fight him off.
*gasps for air*