He is the adopted son of the Architect and the Old One. He is introduced when Arthur meets him in Grim Tuesday. He wields a harpoon made of the luminous trail of a narwhal's wake under the Aurora in an Arctic sea, created by his Mother, the Architect. It is painful to mortals and Piper's Children when used.
He owes a debt to Arthur for freeing him from Tuesday , and therefore gave him a whalebone disc that can summon the Mariner three times and obtains him aid from any sea-faring Denizen, as they all revere the Mariner. All three of the calls are used, once each in Drowned Wednesday, Lady Friday, and Lord Sunday. In Lady Friday it is Leaf who summons the Mariner.
The Mariner wishes to keep himself out of the House's political and other problems, and stated that when he had responded to Arthur's third and final call he would not help him again, except by his own choice. After being called for the third time in Lord Sunday, he sacrifices himself by using his harpoon, which turns into a wave of salt water, to open the cage containing part seven of the will, an apple tree.
He is possibly based upon the Ancient Mariner. This speculation is due to two references, the first one to his remarkably ill-fated shooting of a bird, and the second in his name, Shelvocke. Additionally, like the Ancient Mariner, he wanders the world, unable to die.