Testarossa

Originally a high-ranking demon general known as the "Primordial White," she is a female demon characterized by her silver hair and red eyes. Diablo recruited her to handle miscellaneous tasks, and in Volume 11, she officially became a subordinate of Rimuru, was named, and evolved into a Demon Lord. In Volume 14, she was appointed as one of the Holy Demon Twelve Guardian Kings with the title of "Killer Lord," and in Volume 15, Rimuru transferred his soul to her, allowing her to awaken and evolve into a Demon King. Her name is derived from the Ferrari Testarossa, a supercar.

She is intelligent, proud, and elegantly confident, but she has a bad habit of looking down on others. Despite her delicate appearance, she has a belligerent and cruel personality, and a few years ago, she caused the "Crimson Lake Incident," which annihilated the Kingdom of Silberia under the Eastern Empire. Upon seeing Rimuru, she decided to serve him, and with her aides Moss and Shien, she went to the Western Nations Council as a diplomatic officer and also served as the commander of the Western Deployment Army, which was composed of support troops gathered from various countries. After the war with the Eastern Empire, she delegated the council to Shien due to her original ties to the East and went to the Empire where Masayuki had become the new emperor.

In battle, she excels at precise magic manipulation and uses "Death Blessing" (Death Strike), a nuclear magic that forcibly rewrites genetic sequences and destroys souls, making her as troublesome as Diablo. Through her experiences in the battle with Velgrind and discussions with Diablo, she confronted her inner self and acquired "King of the Realm of Death (Belial)," an ultimate ability governing life and death.

Moss

A demon under Testarossa. A Grand Duke of the demon world with power second only to the Primordial and has lived for tens of thousands of years. With his master's awakened evolution, he evolved into a Grand Duke-level Demon Lord, holding the title of the Empress's confidant. Normally, he appears as a boy in his late elementary school years, but this is because he constantly sends out clones for reconnaissance, and after absorbing all his clones, he takes on the appearance of a handsome man with silver hair and blue eyes. He has the ability to send out "clones" to various places, which he uses for gathering information, and is highly valued by Diablo and Souei.

Shien

A demon under Testarossa. A baron-class demon undefeated for 300 years, his origin is ancient and very close to the Primordial. With his master's awakened evolution, he evolved into a viscount-class Demon Lord, holding the title of the Empress's secretary. Skilled in paperwork, he is often delegated responsibilities by his master, and in Testarossa's absence, he was appointed as the diplomatic officer of the Western Nations Council.