Cas is a fallen angel, Anna-style. He asked permission from Jimmy's mom and told her who and what he is, where he hid his grace, etc etc. He then hitched a ride with Jimmy and became his twin.

He basically lived his entire life as a human, but when he's in his early 40s, Jimmy suddenly falls gravely ill and the current medicine at the time has no answers for him.

Their mom finally reveals to Cas his true nature and tells him about his grace. She tells him the same warnings that Cas gave her when he was still an angel: "You'll never grow old. You'll outlive everyone that you love." Etc etc.

Cas acknowledges the consequences and still takes his grace. His brother can't die yet, Amelia and Claire still needs him. He heals his brother.

They stay happy for a long while (with Cas secretly healing his family's illnesses here and there). But eventually the "I just age better than you" excuse grows tired, and Cas knows he has two choices: reveal his true nature to the rest of the family, or run away. His mother would respect either decision.

He decides the latter. He doesn't want to risk their safety by giving them knowledge as dangerous as knowing his existence. He keeps them in the dark and gives them the "I need to find myself" excuse and starts driving around in his motorcycle.

He eventually falls into hunting. After all, what's the use of this millenia-worth of knowledge if he's not gonna use it to help?

He sends his family letters from time to time but never shows his face ever again. (How can he even explain the fact that he didn't even age a little after a decade or so?) He couldn't even be there during the funeral of their parents, he could only watch from afar.

One day, tragedy strikes: he finds out that Amelia and Jimmy has been killed. (Claire is safe, Jimmy told her to stay in the basement.) Traces of sulfur tells him it's a demon.

He makes sure from afar that Claire is dropped off to a distant relative of theirs before he goes into his vengeance plot. After all, he reclaimed his grace for his brother's sake and it still ends up like this?

After a few years or so, he finally kills the demon (it's another Knight of Hell who caught scent of rumors of a fallen angel and started digging). He doesn't feel fulfilled.

Feeling remorseful for being so deep into his revenge plot that he failed to check on Claire for a while, he goes back to that distant relative only to find out that they're dead. He panics and starts looking for Claire's trail.

He finds out what happened to Claire after all these years (which follows nearly the same path as canon): relative dies, she gets thrown into juvie for a while, she escapes. This time, she meets a hunter while she was looking for who or what killed her parents. She becomes a hunter as well.

Their first reunion obviously doesn't go well. She resents him for leaving her alone at the time when she needed someone the most. He tells her what he is (because he had to assure he that he's not a shifter or a demon) and that he already avenged her parents.

She deflates after finding out that she spent all this time in a revenge plot that was already finished. It takes a long while, but they eventually reconcile.

A few decades pass even more, and even Claire eventually passes away of old age. Eventually, the only one left of Cas' family is himself. He eventually gets tired of the repetitive cycle of hunting.

After quitting hunting, Cas tries on different professions, but eventually settles into being a tailor. He skips town after a few years in order to avoid suspicion due to his unchanging age.

Over the years, he forms a network of connections with other supernatural creatures who just want to live in peace, away from any danger. They're the ones who help him when he needs a new location.