Layla Comet is a character the Author, Molly Grimey, created if she were ever thrown into the life of being a hunter in a different dimension than the original story takes place. In her dimension, it is 2010, and Jack is already born, and the boys are slightly older than they would be in the actual Dimension during 2010.
Layla Comet was born in 2006, however she came out looking the way she does now. She learnt about the world very quickly, and her Father enrolled her into this local school. Nothing seemed odd in this school, until one day, she was walking the halls, and this man ran up to her, eyes yellow, as people yelled chasing after him. After that, she knew nothing would ever be the same.
In the original show, Kline is played by(and also imaged as) Alexander Calvert. In this book, it is the same, for if we didn't have Calvert, the Author wouldn't have taken such a liking to him.
Jack is a boy who was born about six years before the actual timeline, making him six years old on May 18, 2010 in this dimension, and this was one of the only other dimensions that Chuck(God) didn't destroy. Either way, Jack and Layla don't actually know that they like each other, however Layla is more aware of her feelings, seeing as she went to a school to learn how to act like a human, so she knows she likes him, but Jack doesn't understand feelings quite as well, since he's never felt these kinds of things before.
Sam Winchester was only six months old when his mother died on the ceiling. After that, his father put him and his brother through the most horrible things, all to try and get revenge. His mother died, then his girlfriend, then his dad, and even he has died before. By the time he started to give up and want his apple-pie life, a girl he considered his daughter crashed into his life.
Dean Winchester is the oldest of the two Winchesters. He devoted his life to saving his friends, family, and even strangers before saving himself. He was in inspiration to many, and he is only getting started. All his life, he has died and came back to life many times, all of which he came back pretty much the same, except for the first death. However, he stayed kicking, and now he and his Brother are some of the most known hunters in the world.
Castiel, an angel. His powers have helped so many, erased memories of some, and even brought Sam and Dean back many times. He has stayed loyal to the Winchesters ever since he left his angel brethren, and he has made mistakes. He fell for the righteous man, and after he meets the Winchesters, he met his adopted son, Jack. After that, he met the girl that Heaven was talking about, the unknown creation.