The changeling operative most familiar with the Reaver castle, lost before the rescue could use them.
Explaining the Honeycomb Hallways to Carl, Juice Box said the changelings had infiltrated The Reaver Castle a few times, but that they lost the asset most familiar with the place. That loss is why the party had to rely on Doctor Bones' Paper Map and Find Crawler to locate Louis rather than simply walking a known route.
Not described; a changeling operative embedded in the Reaver castle.
The missing expert whose absence forces the party to improvise the infiltration.