The Onimaro Leader appears to be a humanoid Yo-kai with grey skin, covered almost completely by a dark grey cloak.
The cloak is covered in a pattern of various symbols resembling yellow eyes. There is a rip in the section of the cloak covering this Yo-kai's face, revealing a single, giant, yellow eye.
It often shows a cruel personality, willing to make its own fellow Onimaro suffer if it sees fit (namely Ogu, Togu and Mogu). It also views humans with some disdain, much like Rasen, seeing them as tools who, once they are no longer useful, should be disposed of.
Like the name would imply, the Onimaro Leader appears in Yo-kai Watch Shadowside: The Return of the Oni King as the leader of the Yo-kai Virus known as "Onimaro", who aims to use the malice in the hearts of people to revive the Oni King Rasen. Eventually, Rasen absorbs it. As a result, when Rasen is killed, the Onimaro Leader dies as well.
Yo-kai Watch: Wibble Wobble
The name "Onimaro Leader" is derived from "Onimaro", and the English word "leader".