(Capcom's primary storyline)
Diego was a longtime friend of Glenn Arias. On the day of Glenn and Sarah's wedding, a bomb was dropped at the ceremony; killing everyone except Diego, Maria and Glenn, who mourned the loss of his newfound wife. Although he'd survived the explosion, Diego became horrifically disfigured in the destruction and later required A-Virus experimentation as well as other surgical enhancements to survive.
Diego and his daughter Maria then went to retrieve Rebecca Chambers, and as Maria took her unconscious body, Diego utilized an M134 Minigun alongside a troop of armed mercenaries to prevent Chris Redfield and Leon Kennedy from interfering. They then departed in a military cargo truck.
When Chris and Leon infiltrated Arias' headquarters, Diego attacked Chris in the lab. Diego was burned by an incendiary grenade that reacted to the highly flammable A-Virus in his flesh. Surviving this encounter, he later stumbled upon a barely alive Glenn after he fell to the bottom of a skyscraper. Arias told Diego to break his safety mechanisms which prevented him to uncontrollably mutate. Diego and Arias merged to form a large Tyrant-like creature and attacked Chris and Leon. After a long battle on the skyscraper, Chris's B.S.A.A. unit Silver Dagger came to assist them. Nadia killed Diego with a shot to the head from a rail gun and Chris shot a grenade from a grenade launcher into Glenn's heart, killing him and leaving only bits of flesh behind.
Unbeknownst to Chris, Leon, Rebecca and the Silver Dagger unit, Diego's daughter Maria had survived the bombings of the tankers. She returns to Arias' headquarters and finds only her father's surgical metal mask, mourning his death and silently vowing revenge.
Personality and Relationships
Not much is known about Diego's personality prior to his mutation. Arias refers to him as one of his 'dearest' friends. Diego appeared to be very close to his daughter Maria, maintaining a paternal bond with her even after his mutation. He seemed to show concern for Maria prior to her setting out to deploy the gas into the city. Diego was willing to keep Arias alive by initiating the 'final stage' and physically merging his body with Arias'.
Skills and Abilities
Genetically Enhanced with the A-Virus, Diego's physical mass seemed to have been greatly enhanced by the virus making him incredibly strong. He was able to wield a large minigun with ease as well as inflict major damage with his fists, easily smashing through concrete. Diego's immense strength allowed him to swat Chris away with ease and wind him greatly. He also possessed the ability to mutate further, physically merging with Arias to form a new being. However, he seemed to require the aid of medical equipment when not active, hooked up to some sort of apparatus when resting. He was also highly susceptible to fire and was easily taken down by an incendiary grenade. Furthermore, he was completely mute after the accident, his metal mask preventing him from communicating.