The Peacemaker

When Dean is dragged from hell by Castiel, Seraphina takes this as her sign from the lord to join her brothers. Though an angel stripped of her grace and forced to live out the normal human life she is still a sister to her comrades who require her help. At first the Winchesters are hesitant to accept her help but if they want to prevent the 66 seals from being broken they'll need all the help they can get. Along with Cas and Uriel she will assist the brothers in stopping the impending apocalypse and maybe, just maybe help Dean overcome the nightmares that haunt his dreams.

Set from the start of Season 4 so in the words of River Song: SPOILERS!