Not much is known about Diego before he became a vampire, but it is known that he had a goal: to leave his street life and go to college. He managed to stay out of trouble, but when his little brother didn't and died from it, Diego tried to avenge for him. That night, he was cornered by armed enemies until Riley came to his rescue, asking if he needed a new life. Then, he brought him to Victoria and she transformed him.
In the past eleven months, Diego acted as the most responsible member, always looking after his lesser tamed mates, even if they didn't want to be looked after. Part of his obedience came from the fact that Riley had been his only friend since he became a vampire and was very loyal to him.
The Short Second Life of Bree Tanner
- Main article: The Short Second Life of Bree Tanner
Diego became one of Bree's few companions (the other being "Freaky Fred") one week before they moved on to Forks and later became love interest. He was first introduced while hunting with Bree, Kevin, and a blonde kid only referred to as the "Spider-Man Kid". He lectured the others for being reckless while hunting, but warmed up to Bree because she was one of the "tamer" newborns in Victoria's army. Diego was seen as kindly and was appreciated by Riley, and was therefore reluctant to deceive him in order to gain information about the true leader of the army (Victoria, though she is known as "her" by the newborns).
When Diego went to tell Riley about the truth about the sunlight, he disappeared. Bree later realized that Riley implied that he was dead when he returned alone; Victoria tortured and killed him, with the help of Riley, for knowing the secret. Bree was distraught when she realized this, and it was one of the reasons why she did not fight her fate at the hands of the Volturi.
- Main article: Bree Tanner
He quickly formed a close bond with a younger born, Bree Tanner, after they got to know each other better. Diego kissed Bree twice in the novel, and proved to her that the vampire traditions were not real; for instance, vampires don't burn in sunlight or get killed by stakes. Bree initially disliked him, just like everyone else, but warmed up to him when they went hunting together and realized that he was different. Many fans believe that Bree fell in love with Diego over the course of "The Short Second Life of Bree Tanner", while some believe that they were a bonded pair and had a simple innocent platonic, non romantic relationship.
They actually formed a B.F.F secret ninja club, with a secret handshake that Diego died too soon to complete.