A pilgrim who "failed to die as was ordained". He was once a sorcerer in Vinheim. He vows his service to the player and offers to "tease out your true strength", as he is keenly aware that those branded by the Darksign possess "something quite special". He can give you power by using your Dark Sigil.
Even if you don't accept his offer to draw out your strength, Yoel will eventually die after you enter the Catacombs of Carthus, apparently of natural causes. When visiting Firelink Shrine in another time, when it is dark, you will find the Hollow's Ashes near where his body was. The description of the ashes explains that Yoel "faithfully served a woman, only to become separated from her". Presumably, the woman is Yuria, who appears in his place after he passes away.