Asa Fox was a hunter who was rescued by a semi-retired hunter Mary Winchester in 1980 from a werewolf. Inspired by Mary's actions, Asa became a hunter and developed a reputation as one of the veteran hunters.
In 1980, when Asa was just a boy, he was rescued from a werewolf by Mary Winchester. Now aware of the supernatural world Asa was inspired by Mary to go into hunting. Asa was shown to highly idolize Mary and often wrote post-cards to her but was unable to send them to her, as Asa didn't know that she was killed in 1983. His mother however discouraged his hunting and sometimes tried to stop him by misleading him or sabotaging his evidence, but Asa still continued hunting regardless.
Throughout his life he became a notoriously well known hunter. He was known for many outlandish tales about cases he took. Notably, it's claimed that he killed 5 wendigos in one day, and as a result his friends use the mentioning of "wendigos" as a drinking game (a shot every time it's said). He was also a womanizer and often had one night stands. During one of these times he fathered twins Alicia Banes and Max Banes with a witch, who proceeded to raise them herself, presumably for their safety because of his reputation as a hunter. Not even his own mother knew of their relation to him. Though Alicia and Max were aware of their relation to Asa and still loved him as a father, implying that Asa was still somewhat involved in their lives. He also met sheriff and part-time hunter Jody Mills on a case and she formed a deep attraction for him.
During his life he encountered a demon named Jael and exorcised it back to Hell. However years later, Jael returned from Hell and begun a killing spree to spite Asa, and the two became bitter enemies. Jael was known for hanging his victims in the forest for Asa to find. While tracking a lead on Jael with Bucky in the forest, Bucky became scared and wanted to turn back but Asa refused and called him a coward, and the two had a falling out. As they came to blows Bucky accidentally pushed Asa and he tripped into rocks, killing him instantly. Unable to tell the hunter community what really happened Bucky staged Asa's death to appear as though Jael had killed him, and hung his corpse next to a tree.
A service was held for Asa where his kids and mother attended along with a resurrected Mary who was horrified to learn what had happened to him after she saved him. Lorraine showed disgust towards Mary as she'd blamed her for Asa choosing the hunter's life. At the service, Jael showed up and killed several guests and possessed Jody and revealed the truth of Asa's death to them, before being exorcised back to Hell yet again. After the incident Asa and the others killed were given a hunter's funeral and Lorraine forgave Mary after realizing the good he had done, and to learn he had lived his life and had kids as she previously believed he didn't.