Rengokuki

is the original demon who was chosen by Entei as its rider.

History
Rengokuki was one among countless humans and demons who set out to prove their worthiness as rider of the fearsome demon horse Entei, as legend held that he who rode it would rule the world. Rengokuki was the sole survivor of this tournament, and was allowed to ride astride the beast. Some time following these battles, Rengokuki, along with Entei, were sealed away by a powerful priest named Shinsen, and remained so for several decades until the priest was killed by the Infant.

During the story
Released from his seal by Entei, Rengokuki attempted to resume terrorizing villages and conquering the world. He impulsively offered to make Kagura his mate (in Japanese, he asked her if she thought he was impressive before offering to be his bride), but she had no interest in him whatsoever, saying that she didn't think he looked like he could take over the world. After Kagura rejected him, Rengokuki chased after her on Entei in anger and attempted to kill her for insulting him. But before they were able to battle one another, Hakudōshi appeared and said he'd take care of Rengokuki. Rengokuki scoffed at the child, thinking he was no match for him at first glance. Entei then threw Rengokuki off of his back, and stared transfixed at Hakudōshi. This action shocked and stung Rengokuki, who then vowed to destroy the boy to prove to Entei that he was its true rider. Rengokuki swung his halberd at Hakudōshi, but missed him completely; Hakudōshi stood on the tip of the weapon, much to Rengokuki's astonishment. Hakudōshi then easily tore the halberd out of Rengokuki's grip and brutally ran it through his throat, then vertically bisected his upper half with it, killing him. Following his death, Rengokuki was usurped by Hakudōshi as Entei's new master.

Personality
Rengokuki was an arrogant and over-confident individual. Having won constant battles to win Entei's favor gave him a sense of being all-powerful and glorified by others. It was his pride that led to his undoing, as Kagura's refusal to marry him and his desire to punish her for this slight led to his eventual battle with Hakudōshi.

His constant need to prove his worthiness to Entei and others did project a sense of self-insecurity. When Inuyasha overpowered his energy blasts and he told Entei to run, he told the horse his weakness was due to hunger, and that there was no other way he could "lose to a punk like that." When he died, his final words were a lament to dying at the hands of a child.

Physical description
Rengokuki had dark green skin with hay-colored hair. He also wore armor and had a short horn sticking out of his forehead.

Powers & Abilities
Rengokuki was a reasonably powerful demon who was skilled with a halberd. He could also fire energy blasts from his mouth. They had enough energy to knock Inuyasha back when he blocked them with Tessaiga, and set fire to various houses and even hydrated grass.

Relationships
Rengokuki won countless battles to win Entei's favor, but still felt the urge to constantly please it and make excuses for his shortcomings. Though he would often claim he was the owner of the fearsome demon horse, the fearsome demon horse owned him.
 * Entei

Manga vs. Anime
Rengokuki was a character exclusive to the anime, created most likely to give viewers an idea of just how strong Hakudōshi was. In the manga, Entei chose Hakudōshi as its rider because he killed Shinsen.