Well, I think I mentioned I was born human and that changed. I died and, according to what the Doctor said, de-existed by falling through a crack in my universe. I was completely erased from time itself. But then something happened and instead, I woke up as a Roman, in a completely different place, with a head full of a Roman lifetime, but the lifetime of Rory Williams nearly two thousand years in the future.
The Doctor said that nothing is ever truly forgotten. There's still something left.
There was a trap set up, for the Doctor. But the Doctor is a very, very clever man, so not just any trap would work. So the enemy used Amy, his Companion, as a basis for their trap. Let her mind make the believable scenario.
One of those beings who set up the trap was the Nestene. Basically a collective psychic consciousness. They read Amy's mind and made people, people who didn't know they weren't real, to fool the Doctor.
But Amy hadn't forgotten me. Not entirely and more than that, Amy grew up with one of those cracks in her wall, making her head different. And because of that difference, when the Nestene were making duplicates to fool the Doctor, they got something a bit extra.
This 'body' is a lump of plastic with psychic software to seem human to fool the Doctor. There's the personality of a typical Roman Soldier. Because they didn't plan on there being something else to make it more. That something else was me.
I don't sleep, I don't eat, I don't feel pain because I'm just plastic. Everything inhuman about me is just the basis for the Nestene's duplicate. Usually, I should be just a program running on automatic until the Nestene decide to order me around. But because there was more than there programming, me, it's not that simple. I can fight and I can continue to exist even after the Nestene is no longer existing in time.