The Foul race covers a spectrum of demons that are considered to be gruesome or lowly by nature. Foul demons are often those with amorphous forms or origins, ghosts or the undead, as well as humans who lead violent, criminal lifestyles (such as Yakuza members) or have been turned into demons themselves. Notably, a couple recurring demons in this race are based on monsters originating from the Dungeons & Dragons tabletop RPG or are based on American pop culture or urban legends. Demons commonly associated with the Haunt race are sometimes classified as Fouls in some games.
The Japanese name for the foul race, 外道 (gedou, lit. "outer path") is a Buddhist term that refers to heretical practices. Over time, it also came to be used in Japanese as a pejorative term roughly meaning "wretch" or "brute," which is where the term's use in Shin Megami Tensei likely comes from.
In the Shin Megami Tensei series, Fouls are of the Dark-Chaos Alignment and thus cannot be recruited through Negotiation.