The third son of the Architect and the Old One. He was cast into Nothing when he attempted to free the Old One. It is revealed in Superior Saturday that it was Saturday who had the Piper thrown into Nothing, though the Piper thinks it was Lord Sunday. He used his powers to build an army of Newniths; that are extremely organized and intelligent, described by the Piper as "almost denizen," but more like mortals, and he notes that he probably made them too well, as (while they make excellent soldiers) their interests lie in agriculture. He intends to use them to claim the House. He claims that Sunday betrayed him, whereupon Part Four of the Will contradicts him, reminding him of his attempt to free the Old One.
In Sir Thursday, the Piper invades The Great Maze, retreating only when the combined forces of Lord Arthur and Dame Primus fight back. He is alive and wounded from the Will's acid spit, but his current whereabouts and intentions are unknown. He claims that he should be the Rightful Heir as he is the most like the Architect, though the Will refuses to acknowledge him. It is worth mentioning here that his Mother, the Architect, clearly wrote in her Will that only a mortal could be the Heir.
It is the Piper who brought the Pipers Children and the Raised Rats to the House, and is able to control them with his Pipes. By the end of Sir Thursday, Suzy and Fred are put under his control during the disastrous raid on the Nothing Spike. The fact that he controls both Rats and Children with his Pipes quite obviously identifies him as the Pied Piper of Hamelin. In Lady Friday, he leads some of his warriors (Piper's Children) to Friday's Scriptorium in an attempt to get the Fifth Key before Saturday's forces or Arthur can reach it. He commands Ugham to take the "Fifth Key", which turns out to be a fraud. Ugham is destroyed, and the Piper barely escapes, entering the Improbable Stair with his Pipes as the necessary focus.
In the beginning of Superior Saturday , it is revealed that the Piper has withdrawn from the Great Maze, causing damage with bombs made of Nothing. His New Nithlings have moved aboard Raised Rats' steamships on the Border Sea and have reached the Upper House at the bottom. Toward the end of the novel, it is stated that the Piper intends to attack the Upper House.
In Lord Sunday , The Piper launches an all-out attack on the Upper House, quickly overwhelming Saturday's forces and advancing up the Tower at an alarming rate. When Suzy, Leaf, and Doctor Scamandros arrive to stealthily activate the elevators, the Piper has pursued Saturday to the Apex of her tower, greatly aided by the withering of the Drasils. He launches his Newnith army against Saturday and Sunday, quickly allying with Saturday when his forces overwhelm hers. A joint army made up of the Sorcerous Supernumeraries and Newniths attack the Elysium but are turned back by Arthur using the Seventh Key to halt the battle.
When Arthur, Sunday, the Piper, and Saturday meet at the Will's Cage, he reveals he is nothing but a ghost of his former self, held together by his own willpower. When the Piper asks Sunday who cast him into the void, Lord Sunday reveals it was Saturday; in response the Piper stabs her with a sorcerous blade and kills her. Even after Arthur claims the seventh key, the Piper attempts to control Arthur using his pipes, though is prevented from doing so by Lord Sunday. He is quickly dispatched by Arthur, leaving only his golden mask.