The Janitors of the Arcane Helix (alternately, Helical Janitors) were an elite force of operatives loyal to the Loremaster of the Arcane Helix. Judging by their name and traditions, the Janitors arose from actual cleaners in the magical research facilities of the Arcane Helix, a position which naturally involved dealing with unusual, and often hazardous, messes. They were characterized by their loyalty and elaborate traditions of martial honor.
In PRPG1, the Janitors were apparently extinct, but various evidence of their existence was seen in the ruins of the Arcane Helix, including an elaborate training course which the party flunked.
In the infamous PRPG one-off Special Agent Shiny Buttons, the Janitors bested the player characters, rendering the outcome of the one-off non-canonical.
In PRPG2, the Wizard-Lords of Nevershyed (or at least Numinous Spirit) also had a unit of "janitors" who might have been carrying on traditions of the Helical Janitors. The original Janitors were also seen or referenced in various flashbacks involving the Loremaster Eldritch Lore.
In PRPG3, Clean Whistle appeared to be part of a similar janitorial tradition. Tune Tome, speaking of her, said "I thought she was one of those ninja-janitors," suggesting the concept continues to have some currency in the culture of that era.