Diamante is one of the three top officers of the Donquixote Pirates. He occupies the Diamond seat of the Donquixote Family and is the leader of the crew's Diamante Army subdivision.
He was the proprietor of the Corrida Colosseum of Dressrosa, where he earned his title as the "Hero of the Colosseum" Due to his actions and role, he is one of the major antagonists in the Dressrosa Arc.
Diamante was very poor as a child, and grew attached to a pirate outfit that he found, wearing it to feel important as he dreamed of becoming a champion. He ate the Hira Hira no Mi at some point.
Thirty-one years ago, a fourteen year old Diamante had befriended young Doflamingo along with a group of others which consisted of Trebol, Vergo, and Pica. Impressed with Doflamingo's Haoshoku Haki, they pledged their loyalty to the former Celestial Dragon and treated him like royalty, protecting him from danger and training him in the deadly arts in order to make him the Pirate King. Their devotion for him is such that they would kill any and everyone who would so much as offend Doflamingo in even the slightest manner, going as far as burning down an entire town for having poor pavement that made Doflamingo trip.
Back at Spider Miles in the North Blue sixteen years ago, he already was an officer of Doflamingo's crew. After Law applied the crew, he and Trebol spoke with him about the remainder of his life and Corazon's behavior as Corazon threw Law out the window. After Law had been with the crew for about a week, Diamante was seen having a meal with the other members of Doflamingo's "family". Diamante wondered how long Law would last in the crew. They then learned that Law was sick with the Amber Lead Syndrome.
After Law was made an official member of the crew, the Donquixote Pirates spread their influence as they traveled to the Grand Line. They participated in various activities such as looting, making business deals, and collecting bounties. During this time, Diamante taught Law sword fighting techniques.
One day, Corazon suddenly left the crew and took Law with him. Six months later, Diamante had a meeting with Trebol, Pica, and Doflamingo. They noticed that the Marines were not tracking them during Corazon's absence. Diamante commented that it might be a coincidence. The crew also made plans to steal the Ope Ope no Mi during a trade that was going to occur on Rubeck Island. They also planned to meet Corazon on Swallow Island three days before the exchange.
The crew later arrived at Minion Island where the pirates who had the Ope Ope no Mi were based. Shortly after their arrival, they were informed of Corazon's treachery by Vergo. While Doflamingo covered part of the island in his birdcage, the Donquixote Pirates raided and looted the Barrels Pirates' hideout. When they found Corazon, they viciously attacked him until Doflamingo arrived. Buffalo and Baby 5 later informed Doflamingo that the Marines stationed at the island took custody of a boy. Thinking that the boy might be Law, the Donquixote Pirates left Corazon for dead and prepared for departure. Before they left the island, they found themselves under fire by Tsuru's squadron.
One year later, the Donquixote Pirates held an auction for the Goru Goru no Mi, though Doflamingo did not intend to actually sell it. Gild Tesoro went to the auction and sent criminals to kill many people, allowing him to gain possession of the Devil Fruit. Doflamingo got angry and sent assassins to kill Tesoro, but they all failed.
Two years later, Diamante participated in Doflamingo's takeover of Dressrosa. After Doflamingo successfully made Riku Doldo III a villain in the citizens' eyes, Diamante, Trebol, and Pica stepped in and subdued the Dressrosa army, passing themselves off as heroes. Diamante also pursued Scarlett and Rebecca until Thunder Soldier intervened. While Scarlett was getting food for her daughter, Diamante shot her. Later on, Diamante was informed by Viola that Scarlett was actually her older sister. With this new information, he laughed at how foolish Scarlett was to buy food saying that not eating could have saved her.
As time went on, Diamante had become loved by the people in the Corrida Colosseum, becoming its proprietor known as the "Hero of the Colosseum" and earned a high rank in Doflamingo's army, sitting in the Diamond chair.
He was first seen on Dressrosa, discussing matters regarding the Mera Mera no Mi with Doflamingo, which his captain entrusted to him.
Later he was seen in the Corrida Colosseum, where he was announced after the four other Donquixote Family members as part of the tournament to win the Mera Mera no Mi.
During the tournament, he was watching the battle royale from the staff room above the stands. He was talking with Bellamy about the latter's assassination assignment, saying that Doflamingo was too soft to give him another chance. He told Bellamy that if he got rid of Luffy before the end of the tournament, he would become an officer within the organization.
He was later seen musing about the fight between Lucy and Chinjao, and remarked in surprise about how both fighters could use Haoshoku Haki.
While Law and Doflamingo were fighting on the iron bridge, Diamante contacted Doflamingo via Den Den Mushi and informed him about Violet's betrayal. Doflamingo then ordered Diamante to send Lao G and others to guard the underground factory.
While Dellinger was hindered by Bartolomeo in his attempt to kill Bellamy, Diamante ordered him to forget Bellamy and go to the underground factory.
When Franky attempted to breach the Toy House, Diamante heard the commotion through Den Den Mushi.
He entered the arena along with "Lucy", Burgess, Bartolomeo, and Rebecca. They were suddenly interrupted when some of the disqualified participants from Block D charged in, refusing to accept that Rebecca was the winner. Diamante stepped in and repelled them with the power of his Hira Hira no Mi, telling them to accept their losses. Diamante then introduced a new type of Fighting Fish that could reach contestants in the ring and explained that the Mera Mera no Mi was in a chest on the back of one of those fish. After Diamante declared that whoever could steal the chest and be the last person standing in this round's battle royale would be declared the winner, the match began.
When Lucy jumped on the fighting fish holding the Mera Mera no Mi, Diamante attempted to attack him in order to prevent him from acquiring the Devil Fruit. Much to his surprise, Lucy was able to block his attack and shatter his weapon. After Lucy shattered Burgess's armor, Diamante noted that Lucy's fighting style was different, due to the fact that this was not Luffy.
Later, Rebecca attacked Diamante, but her strike was ineffective because she was using a dull blade. Unimpressed, Diamante retaliated by striking her with a club while saying that she will die if she is not willing to kill. He then mocked her about her mother's death.
As the toys in the audience transform back into their original forms after Sugar's curse broke, Diamante was deeply displeased with Trebol's failure. He was then surprised when Lucy demolished the entire arena with his bare hands.
As the arena collapsed, Diamante was submerged and unable to stop Lucy from acquiring the Mera Mera no Mi. When Lucy's helmet and fake beard fell off, Diamante noticed that he was from the Revolutionary Army. After Lucy destroyed the arena, Diamante used his powers to avoid injury as he fell into the underground trade port. While wondering what to say to Doflamingo, he saw the slaves freed from Sugar's curse charging at him.
He was later seen with Trebol guarding the SMILE factory, having defeated many attackers.
Diamante and Trebol later gathered at the royal palace, which has been relocated to the top of the Flower Hill by Pica. Diamante tried to make an excuse for losing the Mera Mera no Mi, but Doflamingo pardoned him for his failure. After Diamante asked Doflamingo about what they should do about Issho, Doflamingo assured him that the admiral would not be a problem. When one grunt laughed at Pica's high voice, Diamante, along with the other executive officers and Doflamingo, glared at the man angrily.
Later, as Luffy and his allies fought their way to the palace, Diamante stood next to Doflamingo and talked about the time they first met Law. Diamante then waited at the Flower Field and confronted Rebecca once she arrived.
Diamante then chased Rebecca through the flower field, taunting her as he pursued her. He caught up to her and knocked her down with a sword slash. As Diamante prepared to kill Rebecca in the same manner he killed her mother, Kyros arrived and attacked him. Diamante was also unpleasantly surprised to see Luffy and Law after Law removed his handcuffs.
Diamante tried to stop Luffy and Law as they were going to the palace, but Law teleported himself and Luffy away from the executive officer, much to his frustration.
While battling Kyros, the former gladiator gave Diamante a cut on the face. Diamante responded by using "Army Bandera", making the ground ripple.
After Diamante recognized Kyros as the one-legged toy soldier who mourned Scarlett's death, Kyros charged at him again. Diamante was impressed that he can fight with only one leg but warned him that his cape is made of iron. He then aimed an attack at Rebecca, but Kyros managed to deflect it. Rebecca decided to fight too and Diamante encouraged her so he can kill her too. Diamante grinned as Kyros promised Rebecca that he will ensure that she will never have to use a sword again.
While Kyros struggled to keep his balance due to the Army Bandera, Diamante tried to attack Rebecca again, but this time, Robin stopped his attack. Diamante then became infuriated at Robin's interference. Robin berated Diamante for taking advantage of Kyros' handicap. In turn, Diamante laughed at Robin's comment, saying that there is no fairness in battle.
Diamante then pressed his attack on Kyros, causing the former gladiator to become worn out. Deciding that it would be tasteless of him to directly finish off Kyros, Diamante used "Hira Release" to fire paper up into the air, which he used his Devil Fruit powers on to turn into spiked iron balls. The balls rained down on Kyros, Rebecca, and Robin while Diamante protected himself with a metal umbrella, but Kyros batted away all the balls that fell his way, much to Diamante's surprise. However, Diamante shot Kyros in the leg, causing him to fall and get hit by the spiked balls. Diamante then taunted Rebecca to come fight him, intent on killing her. However, Kyros managed to stand, not allowing Rebecca to go near Diamante. Kyros then rushed at Diamante, shocking the Donquixote executive, and the two dueled briefly before spacing out. Diamante then tried to finish off Kyros by using "Hangetsu Glaive," but Kyros broke through his attack and his sword, cutting him down. As he fell, he hit his head on the pole marking Scarlett's grave, knocking him unconscious.
After Doflamingo's defeat, Diamante was later arrested by the Marines and was put in Seastone handcuffs.
But somehow in the Legacy, that some people in the Mushroom Kingdom, especially Francis made funny comments and remarks for his appearance being Steven Tyler, which that made Diamante almost trip and fall comedically. which sometimes the audience would laugh like in a sitcom.
He is reprised by Brad Hawkins in the English dub and Hideyuki Umezu in the Japanese dub.