Millie Winchester - Castiel

"Millie? Is that you?" Dean's hoarse voice said as he opened his eyes.
"The one and only, brother"

Millie Winchester, the older sister of Sam and Dean Winchester, daughter of Mary and John Winchester.

Ten years ago, Millie Winchester died on a hunting trip with her father. Ten years after her death, Millie meets Dean in Hell.
What will happen when Millie is brought back to life when Dean is saved by Castiel?

Castiel pairing - slow burn
Starts in season 4