Sister Mary Constant was a nun who was considered a righteous mortal.
Sister Mary Constant was nun who worked and lived in a nunnery. By the time of her death, she was eighty-three. She was noted to have spent "eighty-three years of quiet, humble, nun-like goodness." As a result, she was considered a righteous mortal.
After learning from the Leviathan tablet that they need the Bone of Righteous Mortal Washed in the Three Bloods of Fallen in order to defeat the Leviathans, the Winchesters begin searching for the ingredients, but are left at a loss as to where they could find a righteous bone. Eventually, as a joke, Dean suggests checking a nunnery crypt.
After arriving in the crypt, Sam searches through a log book of who is buried within and comes upon Sister Mary Constant. Dean agrees that she seems to be a good choice and the Winchesters rob her grave to get the bones that they need.
When the Seraphim Castiel arrives at Rufus's cabin, he examines the bone used by the Winchesters to create the weapon and recognizes it as belonging to Sister Mary Constant. Castiel states that she is a good choice for the righteous mortal they need.