Ultima

Ultima (ウルティマ), also known as Violet (原初の紫), is a powerful Daemon recruited by Diablo. She serves as the Chief Prosecutor in the legal department of Tempest's government. She is a leader of the Black Numbers and is counted as a member among Rimuru's Twelve Guardian Lords.

Appearance
Ultima takes the appearance of a cute and beautiful girl. She has middle length purple hair tied into a side ponytail, as well as purple fingernails. When she's off the clock she wears a variety of purple-colored dresses as casual wear and when she's serious she wears a black skirted military uniform with an armband on her left arm.

As a Daemon, she has the characteristic horns and wings of their race, something she usually keeps hidden. She has six bat-like wings on her back that she can freely shapeshift into whatever she desires.

Personality
Ultima, on the outside, appears to be a simple-minded and carefree princess-like person; but on the inside, she's an extremely calculative and scheming person who's always planning her next move. Befitting a Daemon, she's extremely cruel towards her enemies, preferring to capture their souls and torture them rather than outright kill them, which she gets a sadistic pleasure from doing.

Though Ultima takes joy in the suffering of others, she feels an even greater pleasure when those she tortures or observes struggling, overcome their adversity. She's fond of such individuals, and is even willing to reward or lend a hand to them out of gratitude for entertaining her. One notable instance of this is when she revived Towa as a sort of apology for the actions of her subordinate, who violated the terms of their contract.

Background
Ultima, like her other primordial brethren, was born during the dawn of time before the creation of heaven and earth as the demonic counterparts to the Seven Angels of Origin. Her and the other primordials ruled Hell undisturbed for an eternity with Rouge as its overlord. When Bleu and Vert were resurrected completely loyal to Rouge after their attempt to overthrow him, Ultima, Blanc, and Jaune agreed to never fight each other directly, instead preferring to outsmart each other in things like games and who could incarnate in the material world first.

Many centuries before the start of the story, after having countless failed incarnation attempts, she was summoned by the then queen of Raja. The queen, mistaking her for a goddess, asked for a way to save her people. Ultima in turn gave the queen a tiara that could be used to perform miracles, but whenever it was used, it would drain the user's life force and stain their skin in the color of her magic. The two formed a contract where Ultima would win the game should the queen or any of her descendants be completely stained in her color, while the queen would win if that didn't happen. Ultima's intention was to claim the body of whoever became fully stained as her vessel and sent Lacua to observe the kingdom's royal family.

One day Ultima was approached by her fellow Primordial Noir. Noir, now going by the name Diablo, "coerced" Ultima into serving his new master alongside him. Ultima went along with it, only intending to half-heartedly lend her strength for some years until she got bored, only to discover that Diablo's new master was indeed someone worth pledging her allegiance to.

Trivia

 * She was named after the Ultima GTR supercar.
 * She addresses herself with "boku（僕）", a somewhat childish pronoun that is more commonly used by young boys.