Every day was painful beyond anything I'd ever known. William had warned that he was going to push me but I went to bed each night with every muscle in my body aching. We ran, we boxed, we fenced, we swam. He kicked me again and again until I could absorb the blows without flinching. He taught me how to sweep a room, how to move quietly, how to blend into a crowd.

"Remind me why we're doing this," I said as I tried not to throw up after a five mile run that was more sprinting than anything else.

"Because Carl needs an enforcer."

That was what I was supposed to become. Of course, in the end, things went very differently but I didn't know that at the time. All I knew was that each day was making me stronger, faster, fitter.

Whenever things got tough, I only had to think of her, of the look on her face when our fingers touched, to keep me moving, to give me a fresh burst of energy.

I slept in the spare room at William's house. On the bedside cabinet I kept her necklace. It was the last thing I saw each night and the first thing I saw every morning. I was doing this for her. I was getting stronger for her. I would get revenge for her. Once I felt I was trained up enough, I would walk away, get the train back to London and find her captors. It wouldn't matter how well guarded they were. It wouldn't matter where they were. I would find them eventually, I knew that as I knew that the sun would rise each morning.

At night, my muscles aching beyond belief, I would sleep deeply. In that sleep I would often dream of her, the long locked away memories breaking free from their captivity and coming floating back to the surface once again.

I often dreamed of the times we played together. I grew up in a slum building in the poorest part of Tower Hamlets. In the block of flats where I lived, there was little to make anyone smile. But I had her.

She was a neighbour, of sorts. Her family were on the same floor. I could picture it perfectly in my head. Open my door, ignore the drunken yelling of my mother and whoever was in there that week. Step outside onto the concrete walkway. Walk down a few feet, past the squat, past the smashed window. Then around the corner. More often than not she was there, getting away from her parents, sitting alone and waiting for me.

We made our own games. Five years sitting together on that walkway. We were the same age. I only knew I was six when we met because she told me. She had been in the flat all her life, had never been down to the ground floor, never been outside. Social services didn't even know she existed. We were forgotten people.

We would play little games together. We even invented our own sign language. That way, no one heard us, no one came investigating the sound of children playing. We did it without words. A finger up, time for hide and seek. A flick of the wrist, let's be spacemen. It was the happiest time of my life, playing with her. She was the only good thing in my life. The only pure thing. Everything else was corrupt and dirty. I went to sleep thinking of her, of running away with her. One day, we'd do it, I told her. One day we'd run away somewhere together, start a new life, somewhere no one would find us. It was a dream but it made us both smile. Our innocent little games kept us both from falling into despair at what our lives really were.

But it couldn't last. Nothing good ever lasted. My dreams would turn when I thought about that, sometimes I would wake up in a cold sweat, as if it had only just happened, the images vivid before my eyes.

We were ten. Her father, the useless drug addict that he was, came out of the flat and grinned at her, showing his missing teeth. "I've just made us a fortune," he said, leaning down and kissing her forehead. "And it's all thanks to you, Rebecca."

I had sunk into the shadows. We had been caught together a couple of times and it hadn't ended well. The bruises took a long time to fade.

Neither of us knew what he meant but we found out the next day. I had woken up to find my mother slumped on the sofa in the lounge. I kicked through the empty cans on the floor next to her and found a slice of pizza left in the box from last night. It served a more decent breakfast than I often got. I headed out the door, hoping Rebecca was out there.

She emerged from her flat just as I stepped out but she wasn't alone. She was gripped by the hand by a tall sallow man with greasy hair. The man took one look at me, decided at once that I wasn't a threat and then looked away again.

Rebecca was muttering something, her eyes glazed over. What had she been given? I knew that look, it was the one my mother got when she had a fresh dose.

Rebecca's father was standing in the doorway looking out. "Enjoy her," he said, grinning wickedly.

"I intend to," the man replied.

I didn't know what was happening but I knew it wasn't good. I ran at the man, tried to grab Rebecca's hand, pull her away from him. "Wake up," I begged. "Come on, Bex, wake up."

The man picked me up by the scruff of the neck and threw me backwards along the walkway. "Keep your hands off my property," he said before turning and walking away. I went to run after him but Rebecca's father got a punch into my gut and I was left gasping for air, unable to do anything but stagger. He laughed as he shut the door on my face, the laugh echoed by the man walking away with Rebecca. I tried to go after him but I fell and by the time I was up, she was gone down the stairs.

I ran, desperate to catch up. I saw them in the stairwell, pushing open the door to a waiting car. I got to it as it screeched away, leaving me standing screaming her name.

That would be when I woke up, with the car driving away and me left without my only friend, the only good thing in my life gone.

So I kept training. I did everything William told me to. When he decided it was time for martial arts training, I nodded. When he showed me how to conceal a gun and draw it faster than my opponent, I copied him. When he demonstrated using wire to cut a throat, I committed it to memory as I did everything else.

Her being driven away. Our fingers touching. My fall from the window. The flash of the shot. Soon it would be time for payback. They didn't know what was coming.

There was one night when William got the truth out of me. He never mentioned it again and neither did I but he made me take an oath. With a drop of my blood on a piece of paper, there was my oath. He handed it to me to read. "I will not seek revenge until I am told I am ready."

"When will that be?" I asked, handing the paper back to him.

"That is not something that can be easily answered," he replied. "But I will keep you no longer than is necessary to complete your training. Then you are free to do as you wish."