The Empty Fang (虚空牙, Kokūga) is a race of extraterrestrial beings, far above humanity in terms of technology or power, appearing recurringly in the Kadonoverse. Ever since the early stages of human society, the Empty Fang have observed Earth and judge if humans deserve to continue on living. Thousands of years later, during the Fourth Civilization, after humans developed technology powerful enough to conquer the universe, the Empty Fang fought back, chasing humanity back into the Earth.
During the Fourth Civilization, humans created weapons known as Nightwatches in order to fight back against the alien superpower. The Empty Fang are also directly responsible for the creation of Wonder Bearers, because of their experimentation on the Hero of Humanity, Mylow Starscraper. (This fact, however, is contradicted by the existence of Fay Risky, as she was called a Wonder Bearer, despite living so many years before Starscraper or the Fourth Civilization.)
In the case of the Fangs sent on Earth, their appearances are mostly human. Shared between a majority of them are bright yellow eyes, and silver hair, seen in the case of Ameya and Echoes (in the anime), as well as the two humans with Empty Fang genes, Kousuke and Sora Saiga. These traits, however, are not shared by Brick, who, instead, has bright red skin, black eyes with white irises, as well as curly black hair.
Empty Fangs Sent on Earth
During the Third Civilization, the contemporary era, four Empty Fangs were sent to Earth in disguise, in order to judge humanity. One of these four was captured by the Towa Organization, while the other three remained free. Each one got their own "sanctuary" on Earth, to count as a sort of base for them.
- Echoes, Empty Fang sent to Earth to judge humanity, who was made so that he could only repeat back words he hears, in order to not leak any information about his origins. Captured by the Towa Organization. Basis for the Manticore and Pearl.
- Brick, childlike Empty Fang sent to Earth to judge humanity. Initially appeared in the form of a fetus.
- Ameya, Empty Fang sent to Earth to judge humanity. Also known as the phantom thief Paper-Cut, a thief who steals seemingly unimportant objects.
- Unnamed Empty Fang, sent to Earth to judge humanity. The father of Kousuke Saiga and Sora Saiga, who unintentionally impregnated Mahoro Saiga after she wondered into his sanctuary, the Trail of Fangs (牙の痕, Kiba no Ato). Details Unknown.