The Grey Heralds were once colleagues of Gaster's, in his time as Royal Scientist. They perished, or were presumed so, in the same accident that caused Gaster's demise. They have been rumored to be seen around the Underground in certain circumstances, likely continuing their research with their employer in the world beyond.

Goner Kid, The Doubter, Decapitus, Clam Girl, and The Grin comprise the Grey Heralds, and often disappear as soon as they are noticed, speaking in rhymes and riddles as they fade into the shadows they now call their home.