Rubedo

Rubedo (ルベド) is the youngest sister of Nigredo and Albedo. She is considered to be The Strongest Entity of Nazarick.

Appearance
While Rubedo's exact appearance is unknown, it has been implied that she is some sort of mechanical being.

Personality
Not much is known about Rubedo's personality, but Nigredo claimed that she's not the type that others can easily open their hearts toward. According to Albedo, like the other NPCs, Rubedo is absolutely loyal to Ainz and would likely refuse to take orders from anyone else.

Background
According to her sister Nigredo, Rubedo was created by Tabula Smaragdina using a unique process that differs from the other NPCs he made.

The Dark Warrior Arc
Demiurge indirectly told Albedo about the situation with her younger sister (Rubedo) as she cannot be deployed without Ainz's permission, even during the state of emergency.

The Bloody Valkyrie Arc
Albedo and Nigredo had a conversation about their youngest sister Rubedo. Nigredo warned that Rubedo will one day bring a calamity to the Great Tomb of Nazarick, but Albedo replies that she would never do such a thing. In contrast to Nigredo's feelings about it, Albedo only sees Rubedo as an adorable younger sister who can cause no harm or trouble to Nazarick. However, with her position as an Overseer, Albedo decides to take Nigredo's warning and concerns into consideration despite how she feels toward the NPC.

The Invaders of the Large Tomb
For the possibility that the other Supreme Beings found their way into the New World, Albedo put forth a plan to create a squad to search for them. Albedo asked Ainz permission to take command of Rubedo in her squad. At first, he rejected the idea, saying that the experiment with Rubedo has been more or less successful and that he doesn't plan to move her for the time being. When Albedo said that she wanted to create the strongest team, Ainz burst into laughter, because he thought it was a childish yet understandable explanation. In the end, he allowed her to recruit Rubedo anyways after hearing the Overseer's reason.

The Witch of the Falling Kingdom Arc
Albedo noted how her younger sister Rubedo would probably continue staying by her side even if she ends up finding out about her unknown plan set forth for the secret unit they're in. That, however, according to the Overseer, was simply because of her master Ainz had given Rubedo the order of listening to and obeying Albedo’s commands.

Abilities and Powers
Rubedo is the most powerful NPC that can overwhelm Ainz Ooal Gown, and even Touch Me with full equipment. Rubedo was also one of the four top-rated close combat specialists NPCs (Cocytus, Albedo, Sebas Tian) and of course, she happens to be the strongest among them. Ainz expected that he should give a World Item to "Them" on the 8th Floor to beat Rubedo. However, Ainz stated that no matter how strong Rubedo was, she would not be able to fight one of "Them" to a draw. Particularly, the only ones who can defeat Rubedo are the other denizens found on the 8th Floor with her.

According to Maruyama, Rubedo is viewed as being a superior combatant to Touch Me in the same way as to how Shalltear was said to be the likely winner in her match with Ainz.

Ainz Ooal Gown
As the strongest being in Nazarick, Ainz expects that he should give a World Item to “Them” in the 8th Floor to beat Rubedo in case she was to one day turn against Nazarick. Ainz commented that the experiment to start up Rubedo has been more or less a success, and that he doesn't plan on moving from her for now. When Albedo requested Ainz to allow Rubedo into her squad, he initially rejected the idea, though he later changed his mind.

Despite Ainz's caution towards Rubedo though, according to Albedo, she remains absolutely loyal to him, just like the other NPCs.

Albedo
Rubedo is Albedo's younger sister. Unlike Nigredo who despises her, Albedo thinks Rubedo is adorable and that she would never be a scourge to Nazarick. After the discussion she had with Nigredo, Albedo did not consider the possible scenario of Rubedo betraying the 41 Supreme Beings will necessarily be a bad thing on her part. She would later ask Ainz permission to get Rubedo in her squad, which he allowed.

However, despite her positive view of her younger sister, Albedo was sure that Rubedo would refuse to follow her commands unless Ainz ordered her to listen to her.

Nigredo
Nigredo is Rubedo's older sister. Nigredo despises Rubedo and calls her Spinel. Although Rubedo is her younger sister, Nigredo views her as a potential threat to Nazarick, because she was not made the same way as the others. She claimed that Rubedo is definitely not the type that others can go out of their way and befriend. She believes Rubedo has greatly deceived their sister Albedo.

Trivia

 * In the Web Novel, Rubedo doesn't exist, along with her sisters.
 * Rubedo is a Latin word meaning "redness."
 * The name "Spinel" is the derogatory term for Rubedo by Nigredo.
 * It is very likely that the original position of Rubedo is in the 8th-floor, considering that the 8th-floor is guarded by the most powerful NPCs.
 * During Ainz's conversation about Rubedo, he states that the experiment to "start her up" was a success, further alluding to and possibly hinting to Rubedo's true nature.

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Rubedo