Hazel had been traveling with the Winchesters for almost five months now. She went with them everywhere, on every hunt, to every motel. She was only fifteen years old, but she acted like an adult. The brothers found her on a hunt, and she had no family.They couldn't just leave her behind, so they took her with them. Sam and Dean grew rather fond of her. She enjoyed staying with them, too. 

In just a few months, she knew enough about hunting to go on one by herself. Dean insisted. She knew he way around a shotgun and she could handle a knife. Sam taught Hazel how to drive, and she was the only person besides Sam or Bobby that Dean trusted to drive the Impala. They had traveled to England to investigate a case. They were going to visit their dad's friend Sherlock