This does contain some spoilers
Cain, also known as The Father of Murder, was the first born of Adam and Eve, the brother of Abel, as well as the original knight of hell. Cain and his brother are also the direct ancestors to Sam and Dean. Cain became the worst demon to ever exist before retiring for love. After coming out of a 150 year retirement to fight Abaddon's forces, Cain fell back under the influence of the Mark of Cain and began slaughtering his descendants leading to Dean killing him.
He did play a big role with the Winchesters. He helped save the day but Dean had to make the ulimate sacrifice.
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