Sly

Initially found in a small hovel in the room accessible after defeating Gruz Mother in the Forgotten Crossroads, slowly succumbing to the infection. Talking to him breaks him out of his daze, and he will return to Dirtmouth to open his shop.

If the Shopkeeper's Key is brought to him, more of his goods will be unlocked.

If you visit Sly after giving him his Shopkeeper's Key and learning the Nail Arts of all three Nailmasters - Oro, Mato and Sheo, you will be able to open his storeroom and find him inside. It is revealed there that Sly is in fact the Great Nailsage who taught all three Nailmasters. He rewards the Hollow Knight with the Nailmaster's Glory for their accomplishments.

Trivia

 * According to a livestream Q&A, the development team originally planned for Sly to be a difficult boss who wielded a nail approximately the size of the other Nailmasters'.
 * Sly being more than he seems is repeatedly foreshadowed. For instance, his very first line - "Oro, you oaf... you wield your nail... like a club..." - refers to Nailmaster Oro.