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He and his brother Armando Mendez are the two most powerful drug kingpins and well known philanthropists in Vice City circa 1984, and the heads of their own crime organization, The Mendez Cartel when the events of the game takes place. The brothers are of Bolivian descent.
Diego was voiced by Ruben Trujillo.
In GTA Vice City Stories, Diego and Armando are first mentioned in mission Brawn of the Dead by Jerry Martinez, telling Victor Vance that the shipment of coke that he and his brother Lance had stolen from them belonged to the Mendez Brothers.
Diego and Armando have Vic and Lance brought to them, where they tell them Martinez stole the shipment, and use some pictures of Martinez with the cops and a fake license to prove this to the Mendez's. Vic and Lance keep working for the Mendez brothers until Armando decides to take over all of the Vance's businesses by dumping them in a burning oil storage and leaving them for dead.
Diego, after hearing his brother was killed at the hands of Vic Vance, seeks refuge in his compound in Downtown. Then Vic Vance, with the help of Ricardo Diaz and Phil Cassidy, steals a Hunter helicopter and uses it to raid the compound. He is eventually shot down, but makes it to the compound and after slaughtering many of Diego's men, confronts Diego in the compound's roof. There they are joined by Jerry Martinez and a final confrontation starts where Victor manages to kill both Martinez And Diego for good.
Diego is the more quiet of the two brothers and only speaks Spanish. Reni Wassulmaier implies that he/she had relations with Diego.
- Diego and his brother are possibly based on the character's of the 'Diaz Brothers' a pair of fraternal drug dealers who are mentioned in the 1983 film Scarface and physically featured in it's 2006 video game adaption.
- In Diego's artwork, he is holding a Beretta M12 sub-machine gun, which is absent from the game.
- According to Armando, he and Diego are utilitarians, as well as Diego being a fan of Plato's work while Armando is a fan of Aristotle's work. This is true; Utilitarism defends that mankind should always look towards finding pleasure, while both brothers have become drug dealers for personal gain.
- GTA Vice City Stories
- Blitzkrieg (Post mission pager message)
- The Mugshot Longshot (Boss, alongside Armando)
- Taking the Fall (Post mission pager message)
- Unfriendly Competition (Boss, alongside Armando)
- Farewell to Arms (post-mission pager message)
- Burning Bridges (Boss, Betrayal, alongside Armando)
- Last Stand (killed)
- Armando Mendez - Diego's brother.
- T-Bone Mendez - A character in Grand Theft Auto: San Andreas with the same surname.