The Class-C Brain parasite refers to symbiotic microbes present globally in the frontal lobes of 99% of the human population. It has also been hypothesized by the United Nations (UN) and Takano Hifumi to be the cause of all cultural and religious divisions, and difference in ideologies in the world.

Discovered by the UN decades before Dr. Takano's research into the Hinamizawa Syndrome, the parasite was found to be inside 99% of the human race, but the information was deemed too dangerous and the findings were classified and stowed away. It was later brought forward by the Nomura and the Alphabet Council to the Japanese government during the quarantine of Hinamizawa and its surrounding areas in Higurashi no Naku Koro ni Kaku. Nomura then states that the Hinamizawa Syndrome is only one parasite strain of hundreds present throughout the entire world, and that almost all human beings are infected.

Hanyū further calls those who succumb to L5 her brethren in reference to her tribe, hinting that the parasite is only the physical manifestation of a demon spirit.