Juice Box's late brother, a changeling principal who died on the fifth floor while collecting creature forms.
Trying to explain the impossible apparition, Carl thinks back to Juice Box's brother, who died on the fifth floor. He was a Changeling Principal, and his ambitions in the dungeon ran on two tracks: getting his people off the fourth floor, and touching as many rare creatures as he possibly could in order to fill his "library" of forms — since a Changeling can only become what it has physically touched.
His collection habit is the reason Carl assumes forms are inherited or stored at all, and therefore the reason Juice Box holding The Beautiful Place in her inventory reads as a genuine anomaly rather than a trick.
Deceased changeling collector whose form-library explains, and complicates, his sister's repertoire.