Rubedo

Rubedo (ルベド) is a NPCs and is considered to be the Strongest Entity of Nazarick.

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

Trivia

 * Ainz said that the "operation test" of Rubedo is successful. Rubedo's race may be something mechanical, like Automaton or Golem.
 * 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.