Jei is a demon warrior from Hell. He first appears to roam the land killing anyone in his path. He justifies this by saying he was sent on a mission by the Gods to cleanse the land of evil. Jei travels with a young girl named Keiko, whom he saves from a band of ronin. It should be noted that Jei refers to the child as his "innocent" and Keiko refers to him as "uncle".
Jei wields a spear that can tear one's very soul from their body. Jei's calling card is the utter look of terror he leaves upon his slain victims, out of those he has faced in combat only one has survived: the long eared Ronin Usagi. Jei has faced Usagi several times, yet his most climatic battle between the long eared Ronin was in the Grasscutter story arc. Usagi finds the long lost heavenly blade Grasscutter, which pits him in the middle of a conspiracy to overthrow the shogun. While Usagi travels to find answers, the sacred blade acts like a beacon. the heavenly blade "calls" to the demon spearman in which he heeds, thus leading him to Usagi.
As the battle begins Usagi's friend Gen tries to get the jump on Jei, however Jei quickly takes Gen out of the battle with a wound to the arm. Jei and Usagi face off, however Jei overpowers Usagi. When Jei is about to strike the finishing blow, Usagi picks up the sacred blade and runs it through Jei's chest. In utter anguish Jei lets out an unearthly cry as all the souls of his victims burst out of his body, causing him to disintegrate. when it would seem as if Jei has left the earthly plane, Inazuma is later shown to be possessed by Jei's spirit, where she is soon joined by Keiko who refers to her as "auntie".
Jei is a brutal killer who prefers to wield a Yari spear. In a flashback where he first met Keiko, it was shown that he was able to defeat a group of well armed ronin without any wepons at all. Jei then picks up a spear from one of the slain ronin an enchants it to have the ability to rip souls from peoples bodies.
Jei has shown the ability of possession as he demonstrates at the end of the Grasscutter story arc, as he uses Inazuma as a vessel for his return. Jei has also demonstrated on several occasions to come back from the dead.
On the first occasion it seems he reappears with the same body, while the second time he posseses Inazuma. It has not been revealed as of yet whether Jei is a spirit from Hell that uses the living as a vessel, or if he once had a body of his own before his first apparent death.