Yoel is first encountered near the beginning of the Undead Settlement, at the edge of a crumbled bridge. The only survivor among many other less fortunate pilgrims like him, he seems enlightened to be in the presence of the Champion of Ash and asks if he could be allowed to be at their service. Agreeing to his plea will make him move to Firelink Shrine. There, he can be found on the lowermost level, at the end of the right corridor past Greirat of the Undead Settlement, where he will sell an assortment Sorceries to the player.
Selecting the "Talk" option while in his interaction menu will teach the player the "Beckon" gesture.
Yoel has a secondary mission, which is part of Yuria of Londor's questline. By selecting the "Draw Out True Strength" option, Yoel will allow the player to level up without paying a soul cost, in exchange for also placing a Dark Sigil within the player's inventory. At this point, every time the player dies, their Hollowing level will increase based on the number of Dark Sigils they possess. At 2, 6, 12 and 15 Hollowing, Yoel will again present the player with the option to level up for free, granting them an additional Dark Sigil each time up to a maximum of 5. At this point, the next time the player reloads Firelink Shrine, Yoel will die, and in his place will be Yuria of Londor.
"Please, grant me death, Undo my shackles. Ohh… Ohh, then it’s true… A Champion of Ash, as I live and breathe. To be in your presence is a great honour. I am Yoel of Londor, a pilgrim as you can see, only…Somehow, I've failed to die as was ordained. Well, perhaps my calling lies elsewhere. Say, Champion of Ash, how does the idea of taking me into your service strike you? I was once a sorcerer. Surely I can be of use."
"Ohh. I am honoured, truly. I should be dead, yet you have granted me purpose anew. I Yoel of Londor, do solemnly swear myself to you."
"Ahh, mm, yes, of course. An accursed pilgrim has no place in your honourable service. But if a lost soul should encounter a legend such as you, could that be anything but a movement of fate? I will remain in this spot. Praying solemnly that you might have a change of heart."
"Ahh, Champion of Ash. Have you had a change of heart? I beg of you, take me into your service."
Encounter at Firelink Shrine
"Oh, our Champion of Ash, welcome home. This pilgrim, with a debt in death, hardly deserves to behold this divine flame. And I never would have, had you not taken me into your service. I thank you dearly for this... And assure you of my loyal service."
"As I have said, I was once a sorcerer. Alas, the magic of Londor is a far cry from the wonders of Vinheim. But I can teach you what I know. Perhaps more importantly... I believe that I can help tease out your true strength. We pilgrims of Londor are keenly aware. That those branded by the Darksign possess something quite special..."
"Oh, our Champion of Ash, welcome back. I would do anything for my master, just say the word."
"Draw out True Strength"
"Then, shall we begin? Bearer of the Darksign, let your true strength shine..."
After obtaining five Dark Sigils
"Ahh, you have attained ample strength. All will soon be clear, my good Lord..."
Upon reaching the Catacombs of Carthus, Yoel will die and his services will no longer be available, ending his questline prematurely unless the player has gotten all five Dark Sigils from him.
Yoel dies exactly the moment the "Catacombs of Carthus" banner appears on the screen, so it is perfectly safe to fight the Abyss Watchers beforehand and use the bonfire that appears after the battle, as long as the player does not descend the staircase that opens in that room.