Sadie Elaine Winchester was only a baby when her Mother was killed by a demon. Her brothers Sam and Dean were older when it happened, Dean especially. He was ten years old at the time, and her other brother, Sam, was only six.
Sadie grew up feeling like she was being treated like a 3 year-old all the time. It wasn't because she was the youngest, though that was part of it, it was because she was the only girl in the family. Her father was reluctant to let her train like her brothers, but he eventually relented. She treated her father with a certain amount of respect, but she was always arguing with her brothers, more with Dean than Sam.
Since Sam had left for Stanford Sadie had been stuck with Dean most of the time. She saw her Dad very little.
Dean was reluctant to let Sadie accompany him on any jobs, but he was forced to accept that Sadie was, after all, sixteen years-old, and that when she wanted to she could be an extremely good hunter and fighter. She was also extremely fast as