Joanna Beth Harvelle is the daughter of Bill and Ellen Harvelle. After her father's death, Jo became a hunter in his memory.
Her life is pretty much identical up until Supernatural's fourth season.
Jo encountered the demon, Amy, who escaped before being called in to investigate murders that had been going on for the past ten years.
She questioned Liz, the girlfriend of the latest victim, Ross. She found Ellen also at the crime scene. Liz told them both that Ross' mother and older brother had also been killed. Jo and Ellen learned that Ross' father, Steve, had been with a woman named Sarah Blake, who hanged herself and became a Banshee.
They confronted Sarah who overpowered them before being stabbed with a golden blade by Amy who escaped when Ellen tried to kill her anyway.