Yy was an androgynous angel who fell from Heaven after failing to announce its relocation to all of creation. Instead, they became corrupted and sold their wings for a cheeseburger. Yy was imprisoned on the planet Sheol, location of the tourist attraction Heaven.
They were discovered by Peri, who had been killed by a group of putti so that she could see Yy and convince them to finish their job. The putti restored Peri to life so that she could help Yy close the Gates of Heaven. When that was accomplished, the putti took Yy to the real Heaven, though they didn't get their wings back. (PROSE: The Ruins of Heaven)