Hiyou, also known as Shishigami Hiyou, is a character who originates from the same tribe as the protagonist Kuro. He is a member of the Lion God clan, a group of beings known as Mototsumitama who maintain the world's balance of fortune and power. However, Hiyou is considered a heretic among his own people due to his nature and actions, which stand in stark contrast to the peaceful principles of his clan.

Hiyou possesses a cruel and warlike personality, driven by an insatiable curiosity rather than a sense of duty or morality. His primary motivation is the pursuit of knowledge, specifically regarding the truth about the contract system that binds his kind to human partners. This obsession leads him to commit grave transgressions, such as secretly bringing humans to the Holy Land, the sacred realm of his tribe. He has also conducted experiments on members of his own species, using them as test subjects for his research. These actions ultimately resulted in his banishment from the Holy Land.

His role in the story is that of a dangerous antagonist. After escaping execution for his crimes, he survives and continues his work on the surface world, forming a contract with a human named Shinobu. He was involved in the destruction of the spirit stones in Okinawa nine years prior to the main events, an incident during which he also killed members of the Hiba clan. His experiments are tied to the creation of Tribal Ends, artificial beings that serve as his underlings. Hiyou deliberately approaches the main character, Keita, providing him with hints about the truth of the Doppeliner System, not out of altruism but to use him and Kuro as a means to prevent interference with his own plans.

Regarding his key relationships, Hiyou has a direct and adversarial connection with Kuro. He views her as something to be captured and claimed, and he vows to kill her contractee, Keita. He also has a relationship with Reishin, Kuro’s brother. Reishin, who was responsible for massacring their tribe, saved Hiyou from execution and allowed him to continue his experiments in exchange for cooperation with his larger schemes. Hiyou's contractee, Shinobu, is a critical part of his later plans. He teaches her how to become a Minus Root, a being that can absorb the life energy of others, making her an exceptionally powerful and dangerous partner who does not lose her own life force in battle.

Over the course of the narrative, Hiyou’s development portrays him as a consistently ruthless schemer. He is ultimately defeated in a battle against Kuro and Keita after they achieve a new level of synchronization. During this fight, he receives a severe burn on his left side of his face from Kuro’s powerful Mega Exceed attack and is subsequently incinerated, meeting his end at the hands of the protagonists. His death also leads to the demise of his contractee, Shinobu, who loses her source of power.

In terms of abilities, Hiyou is a formidable fighter. His signature ability is an Exceed technique that allows him to manipulate and control water. This power can be used to apply immense hydrostatic pressure against his opponents or even to cause the blood within a person's body to burst, inflicting severe internal damage. His power is such that he can overwhelm Kuro in combat even without his contractee, Shinobu, at his side.