Endoku is a minor character from the Kishin Douji Zenki series. He is a Hyouijuu that looks like a humanoid figure covered in green acid, that has an exaggerated lemur-like head and large claws. Parts of his skeleton are clearly visible, this especially counts for his chest where his whole set of ribs can be seen.
While his actual backstory is unknown, Endoku first appears after Chibi Zenki has broken one of Ozunu's seals in hopes of getting an easy meal.
Endoku himself mentions that he had been sealed by Ozunu Enno about 1000 years ago and that he's boiling with hatred for humanity and wants to return the favor to them, which probably means that Endoku wants to kill all humans.
While it is unknown why exactly Endoku had been sealed, it soon becomes clear that Zenki broke the seal on "the great tree of Genbu", because he noticed the scent of a Hyouinomi. This makes it very likely that Endoku once served Karuma, thus making him an enemy of Zenki and his friends.
Endoku in the manga
Endoku VS Chibi Zenki
While Endoku at first is just glad that he's free again, he soon enough gets challenged by a little Chibi Zenki. At this point, Zenki obviously has forgotten that he's still a chibi and thinks of himself as the big and strong Zenki-Sama.
Chiaki exclaims that "if Zenki is small, his power is small, too". Shortly after that, Endoku easily swats Chibi Zenki out of the way, who ends up blasting off like Team Rocket.
Endoku then turns to Chiaki, scaring her and burning off her clothes by spitting acid at her. He laughs at Chiaki and runs after her while she flees from him.
Chiaki calls out for Zenki, causing the Bracelet of Protection to glow.
The action cuts to Chibi Zenki, who has crashed somewhere in the forest and is trapped beneath a tree. He starts cursing and now notices how he "was so caught up in the moment that he forgot he was still sealed". Zenki gets furious at Endoku for treating him like this and wants to kill him.
Shortly after this, Zenki hears a ringing noise and notices someone calling him.
We now see Endoku getting blinded by the bracelet's light as Chibi Zenki is teleported to Chiaki's side.
When Endoku notices Chibi Zenki, he laughs at Zenki which causes Zenki to tells Chiaki to return him to his true form. When Chiaki is unable to do this, Endoku beats up Zenki and chars him with his acid.
Chiaki gets really worried, but is relieved when Zenki turns out alright. Chibi Zenki is now so mad that his fury unleashes his true form from his seals.
Endoku VS Demon God Zenki
The way more powerful Demon God Zenki seemingly defeats Endoku with ease, rips a large hole into the beastman's upper body. But Endoku isn't done yet and recovers faster than Zenki can harm him.
While Zenki gets all cocky and doesn't take the beastman seriously, the latter wraps Zenki up with tentacles made of acid. This leaves Zenki in a rather dire situation as he can't do anything.
Chiaki then interferes the fight and uses her "Khan" fire spell. She flings some burning spell tags at Endoku, which don't deal any damage, but distract him long enough for Zenki to free himself and take down Endoku.
Zenki then ends the fight by extending the Diamond Horn from his fist, punching the horn into Endoku's head and shooting him with the "Breath of the Flame King" (aka "Kain ojin" in Japanese).
This kills Endoku instantly, returning him to a Hyouinomi, which is promptly eaten by Zenki.
Anime (unused scene)
While Endoku does not appear in any anime episodes, Volume 7 of the Kishin Douji Zenki manga sports an advertisement for the (then to be airing on TV in the future) Kishin Douji Zenki anime on page 198.
This advertisement features a screenshot of an anime scene showing Demon God Zenki fighting against Endoku in a similar fashion to their battle in the manga.
It depicts Zenki struggling against the Hyouijuu, as Endoku attempts to choke Zenki with his arms and covers him with his acidic body, making Zenki unable to move and forcing him to rely on Chiaki's aid.
Click here to read more about Endoku's forms from Kishin Douji Zenki 3 - Tenchi Meidou.
Kishin Douji Zenki FX - Vajura Fight features an alternate form of Endoku as a boss. In this game he sports a magenta coloration and yellow eyes, yet his abilies remain the same as in the manga.