Fithlings are very thin beings of average height, who must live in containment suits to stay alive. Every Fithling alive was born in the city of Fith and little to nothing is known by outsiders about their practices there. They're very private beings and have a very organized government.
Generally they have no respect for the sanctity or privacy of other races, they care not for the gods, or religion, or magic or demons. Science is all they care for and learning and using it at any and all costs.
They fight wars by using other races and shock troopers, or by poisoning or underminind their foes. They manipulate and only care for themselves and their own progress.
Hood Episode Three
The Hunter encounters a Fithling scientist squatting in a house in the swamp outside of Shadetale. The scientist says he is there to study the mist and demons and is safe from them because the spirits fear technology. This particular Fithling is cruel and racist, though eager to share information with a Fenrirr.
Kingdom of Liars 1
The new recruit encounters a Fithling scientist in a warehouse with a blue door on Whisper Street. The Fithling had ordered a device of Lekling make, but was then betrayed by those who hired him. This Fithling does not care about the slaughter of his employees. Self-preservation is his top priority. Warren brutalizes the scientist to learn all he knows, explaining to the new recruit that Fithlings die if their suits are removed. He also says their helmets protect them from feeling much pain. However, the psychological effects of such physical damage are tremendous. Fithlings are terrified of their suits breaking and allowing in contamination from the outside. That fear drives them crazy.
These frail beings use powerful flamethrowers to even the odds.