The character was an important participant in the events of New Moon, Eclipse, and Breaking Dawn, but readers don’t really get to know a whole lot about the character in the written Saga.
Bewley, however, has a remedy for that: his own back story creation for the character.
In an interview Bewley did with SELF Magazine recently, he explained his idea of a Demetri history.
Source : Twilight Examiner
Demetri lived near Mt. Vesuvius before it erupted–when it did, it killed his family, but he was so fast, he was able to outrun the lava. He ended up in the foothills, where he was enslaved. But he kept running away from his masters, and ended up in Rome, where he became a gladiator. He had no fighting skills, instead, he just ran away from his opponents. Eventually he was blind-sided by an arrow and he went down. Then Volturi leader Aro came in, recognized his skills, and turned him into a vampire–he’s been serving the vampire guard ever since.
You can read the rest and see a short video snippet of Bewley here.