Diego is a tall muscular man with long black hair he keeps slicked back. He usually wears a dark red Lucha Libre wrestling outfit and his laughing mask to show off his happy-go-lucky attitude.
When he fights serious, he removes his smiling mask to the angry mask to look very angry and no longer smiles. Under his mask is a sunglass mask to cover his eyes (possibly because he's too embarassed to show his eyes without his mask on).
Diego is primarily known as the Smiling Steel Fist in the Nine Fists due to his flamboyant and goofy personality. He believes that a fight must always have an audience and that the fight is for the audience's enjoyment, and that after a fight the loser must obey the winner. He has been shown to always be smiling or laughing even when angry. This personality seems to have rubbed off on his disciple. It's very rare to not see him smiling in a goofy or relaxed manner, as when he frowns is usually when he's upset by something or angry with his oponents. He also seems to care greatly for his disciple, Rachel. One of the few times he frowned was during his imprisonment in Big Lock while wondering if she missed him. He even pictured her crying while holding his mask. He's also has a liking for Manga, as he was seen reading one while in Big Lock and was glad his favorite one is still going on.
Despite his comic tendencies, Diego is also shown to be cunning, as shown when he matches up Garyuu X with the Muay Thai Taifun team, which secretly had a 51-year old master in its ranks. During the fight, he pretends not to realize the elder's true identity and enjoys toying with the information when presented with the truth. It is revealed that he lied to the elder about the tournament being for people "20 and under", when in fact the tournament was for those "under 20", possibly as a measure of insurance for invalidating the elder's future battles in the DofD. This takes on a grimmer tone when he places the targets he was sent to assassinate on a ship and fights with Kensei Ma while on the ship, caring not for the well being of those people or the fact that blowing up the ship would send him into the sea as well.
Despite his cunning and goofyness, Deigo does have some honor, as he gladly accepted his defeat at the hands of Ma Kensei and willingly gave him his mask and went to Big Lock.
As a member of One Shadow Nine Fists, Diego is a very powerful fighter. With his dedicated showmanship, he can completely change at will his demeanor from the goofy showoff of Smiling Mask, a Sei-type, to the raging battler of Angry Mask, a Dou-type. With his Lucha Libre fighting style and his ability to maneuver so well makes him a tricky opponent. A testament to his skills is that he, along with Alexander Gaidar, are capable of escaping Big Lock with little effort and can do so at any time they feel like it.
- Master Luchador: Diego is a master practisioner of Lucha Libre, a style that uses powerful techniques, but is ultimately difficult to use in real combat due to its flashly, time consuming, aerial-based, tackling nature of fighting. As a master of this fighting style, Diego has honed his skills in all of these types of fighting techniques, and has mastered them to the point where he can effortlessly use these techniques in battle despite the overall uselessness Lucha Libre has in real close combat situations. Because of his mastery of this fighting style, Diego is an incrediblely complex and unpredictable fighter, and can outmaneveur some of most versatile of fighters, which was proven during his fight with Kensei Ma as he had him on edge during most of the fight, and was so difficult to fight against that Kensei admitted that he couldn't hold back against him anymore if he wanted to win.
- Immense Speed: He was able to move fast enough to make clones of himself. He even managed to sneak up behind Kensei once while both combatants are in the air and delivered a blow to Kensei's neck.
- Enhanced Strength: As a master class fighter and a member of Yami he has above average strength. He is capable of breaking a microphone with just his jaw and tear through stone with little effort.
- Immense Agility: He is extremely agile and can keep most opponents in the air while attacking with many different techniques. Because of his Lucha Libre fighting style, he manages to make sure his opponents don't get a hold of him in the ring. Diego can even maneuver his body out of an opponent's attack before the full force of the blow connected. He pulled this once on Kensei at the end of their fight and managed grab ahold of Kensei when the later has barely finished his moves.
- Iron Defense: Arguably to be the essence of Diego's style, instead of dodging his opponents' attacks he shrugged them off with his enormous defense and continued on with his attacks. He took one of Kensei's signature techniques and was unfazed, which according to Kensei and Renka should have toppled if not killed him, let alone standing. Only when Kensei used techniques that are meant to injure the opponent's interior did he suffered any significant injury. His arm can go through high voltage electric fence without taking any damage.
- Master Entertainer: Diego is a master of entertaining people as he is easily able to entertain large numbers of people, and he always does it with pride. As a entertainer, he is incredibly proud of his "Diego Quality" brand of entertainment and is willing to go to great lengths to turn every important event into a show. He has shown that he is able and willing to use his entertainment as a form of deciption. This was proven during his battle with Kensei as he revealed that his audience were really just targets, and that his real audience were watching everything via surveillance from several miles away from the ship on boats. Despite this, his dedication to showmanship is iron-clad, as even though he lost to Kensei Ma, he did so while ordering his disciple to continue pointing the cameras at him, and even went as far as getting himself and Kensei into a position where all the cameras are right on them, so his audience could see his performance to the very end.
Not much is known about Diego's life before Yami other than the fact that he is quite wealthy, and that he was once a professional masked wrestler. It is unknown how and why he stopped Masked Wrestling, but it was around this time that he joined Yami and became a member of the One Shadow Nine Fists. A couple of years after this he mets Rachel Stanley, and takes her as his disciple. He also participates in the joint training of Sho Kano as the sole successor of Yami.
Diego, as the referee, tried to stack the tournament against the Shinpaku Alliance, but he was so impressed by the showmanship of Siegfried's entrance that he more or less ceased his crooked officiating.
Upon a fiery backlash by the enraged Ma Kensei, Diego loses his composure and takes off his "Smiling Mask", only revealing there to be another mask underneath. This shocks Rachel and causes panic among several of the audience; and Ma Kensei refers to this as Diego's true face; "Angry Steel Fist". Upon this state, DIego battles with Ma with powerful, ferocious attacks, even bringing the battle out of the ring and apparently not caring for the safety of his precious audience in the brawl. It is revealed that he no longer regards these people as 'audience', as they are targets on Yami's list of elimination. The real audience gathered in ships further off, watching the battle via real-time screening.
Diegos tried to keep Kensei in the air, claiming that there is nothing a Luchador can't defeat in the air. However he is defeated when Kensei got back on the floor, saying that the earth belongs to Chinese Kung Fu. Afterwards, his plan was thoroughly foiled, with the intervention of Miu, Kenichi and Furinji Hayato. At the point of defeat of the climax of the battle, he took care to make sure the camera angles were all fixed on him till the end, and declared that it was 'Diego Quality' for him to even lose spectacularly. He offers his mask to Kensei, comically revealing a third mask, and asks to be killed, only to be denied, but Kushinada arrives and throws him overboard. He is saved by elder and sent to Big Lock.Ogata Isshinsai and Tsutomu Tanaka ended, Diego is later seen again inside of Big Lock with Alexander Gaidar. It is then revealed that both he and Alexander can escape Big Lock anytime they want and are staying there willingly because of their pride as martial artists and because they have to obey the winners orders only because they lost to Ryozanpaku fair and square. They stated that they will leave Big Lock only when they have a reason to get out, to which the Yami weapon user told them that the Hachiou Executioner Blade team is gathering again, a statement which intrigues them both. After hearing this the weapon user stated that the One Shadow wanted them to know this and to think carefully about their decision. He is next seen later still in Big Lock, where it is revealed that both he and Alexander have been sneaking out of Big Lock to buy various items out of boredom. He talks to Alexander about how his disciple has found Big Lock and has been waiting for him all this time, then starts to wonder if his disciple, Rachel misses him at all, to which Alexander mockingly asks Diego if he is homesick. When the guards demand Diego give back his posessions, he does so while going through an electric field and warns them if they keep harassing him he's liable to snap.
During the Eternal Sunset with Yami and Ryozanpaku, Diego and Alexander escapes from Big Lock. As the battleships prepare to head out a soldier reports seeing shadows in the water as three of their battleships are sunk. A submarine surfaces with Diego Carlo and Alexander Gaidar inside, who have been guided there thanks to Boris. Both masters arrive to fight along with Ryozanpaku: Diego asks his disciple where he can stand out the most and Alexander wishes to maintain the honor his deathmatch with Akisame and stop Yami's "inelegant" methods.
Fighters arrive to engage the two masters and Diego is accompanied by Meatman and a film crew, preparing to broadcast his battle so he can stand out. Diego asks Alexander why he is fighting against Yami and he answers that if there is no beauty in the Eternal Setting Sun then it must be stopped, as he charges in and mows down a group of soldiers.
Once the Eternal Sunset is lost, Diego celebrates with Alexander.
In the Epiloque, Diego, now pardoned of his crimes, goes back to masked wrestling along side Rachel.
- vs Ma Kensei (lose)