The Nameless Thirteenth is the thirteenth god of the Twelve Gods, although only believed in by the "Right People". He is the god of thieves and crime, and his name is not to be spoken, thus he goes by many titles, among them the Crooked Warden, the Benefactor, the Thiefwatcher and Father of Necessary Pretext.
In terms of commandments to be obeyed, according to Father Chains, the Benefactor only has two mandates, both equally important. The first mandate is "Thieves Prosper" which means that they are to aid one another, hide one another, make peace where possible and generally see to it that each other can flourish and prosper, by hook or by crook. The second mandate is that "The Rich Remember" meaning the rich and powerful need to be reminded that they are not invincible, that they are mortal. Any lock might be picked and any treasure might be stolen and the second mandate is to remind the rich and powerful of this fact.
Locke Lamora is a priest of the Nameless Thirteenth, as was Father Chains before him.