Charlie Bewley will play Demetri. Here's an interview from Hollywood.com (follow the link for the full article!)

Do you actually know Demetri’s backstory, based on what’s in the book?
CB: I made it up.

Tell us!
CB: Demetri was obviously Italian. And at a young age, he was orphaned. He was living in the foothills of Mt. Vesuvius -- the big, volcanic mountain -- and in 900 A.D., it erupts. Lava starts pouring down, and his parents get caught in it. Their house gets taken apart. I outrun the lava, into the foothills, where I found a place where I was safe, essentially. But I couldn’t provide for myself, so I was thrown into slavery. I went through various masters but they couldn’t pin me down, because I was so nonchalant and arrogant and just ran away from everyone; it was really frustrating. I kept getting sold on to the next guy -- the next master, for the best price they could get -- and I was too nice for someone just to kill me.

So eventually I got sold to a merchant trader in Rome, who dealt specifically in fighters, and I found myself in front of the Coliseum with these baying, bloodthirsty Romans, desperate for gladiatorial combat. I didn’t have any fighting skills, so I just ran around for three hours and no one could catch me -- until somebody shot me with an arrow. I was lying on the floor, I was about to die, I was waiting for Commodus’ verdict of thumbs up or thumbs down, and he’s about to give it a down when Aro comes in and whispers in his ear and says, “Don’t kill him, I’ll take him.” Aro converts me in the depths of the Coliseum himself, and I’ve been working loyally for him ever since.