|Next to the Collector's room between Biomes, next to a pile of corpses|
After choosing mutations, the player can have Guillain reset them for 1,000 gold. A second reset costs 25,000 and a third costs 50,000.
Not much is known about Guillain. He sits happily next to the pile of corpses left by the Beheaded’s previous attempts. His proximity to the Collector suggests he knows him or works with him.