Rubedo

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

Personality
Not much is known about Rubedo's personality but Nigredo claimed that she's not the type that others can open their hearts toward.

The Bloody Valkyrie Arc
Albedo and Nigredo had a conversation about Rubedo. Nigredo warned that she will bring a calamity to Nazarick, but Albedo replies that she would never do that and that she only sees Rubedo as an adorable younger sister.

The Invaders of the Large Tomb
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 a success and that he don't plan to move her for the time being. When Albedo said that she wanted to create the strongest team, Ainz bursted into laughter because he thought it was a childish yet understandable explanation. In the end, he allowed her to use Rubedo.

Abilities and Powers
Among all of the denizens of Nazarick, Rubedo is the strongest and most powerful NPC that can overwhelm Ainz Ooal Gown with full equipment, and stronger than Touch Me. Rubedo is also one of the four close combat specialist NPCs (Cocytus, Albedo, Sebas Tian) and of course, strongest among them. Ainz expected that he should give a world item to "Them" in the 8th floor to beat Rubedo.

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 if she one day turn against Nazarick. Ainz commented that the experiment to start up Rubedo has been more or less a success and that he don't plan on moving her for now. When Albedo asked to take Rubedo in her squad, Ainz rejected the idea but later allowed her to use Rubedo.

Albedo
Rubedo is Albedo’s younger sister. Unlike Nigredo who despises her, Albedo thinks she is adorable and that she would never be a scourge to Nazarick. She asked Ainz permission to get Rubedo in her squad, which he allowed.

Nigredo
Nigredo is Rubedo’s older sister. Nigredo despises Rubedo and calls her Spinel. Although she is her younger sister, Nigredo views her as a potential threat to Nazarick as she was not made the same way as the others. She claimed that Rubedo is definitely not the type that others can open their hearts towards.

Trivia

 * 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.