Sometime after the creation of the universe and the evolution of mortals, The Architect had three children as a result of an experiment with the Old One where she inhabited a mortal body and reproduced with the Old One. Although the type of mortal was not specified, it can be safely assumed that it was a human woman since the children all bear the appearance of stylized and exceptionally tall humans.
From eldest to youngest, they are:
Lord Sunday - Master of the Incomparable Gardens and the Elysium, he wields the seventh and most powerful of keys. He has intense eyes and is estimated to be ten feet tall.
The Mariner - A seafarer and patron to sailors of the secondary realms, The Mariner bears a harpoon created by The Architect herself and is apparently capable of crossing between the House and Secondary Realms at will.
The Piper - Despite being the youngest, the Piper is arguably the most powerful and ambitious of the Architect's children. By using his pipe, he can charm all but the most powerful minds, create Nithlings from Nothing, and access the Improbable Stair. After being cast into Nothing by Superior Saturday, he was able to maintain a nearly corporeal form and went on to create his own worldlet, an ability that none of his siblings display.