Ciel

Ciel (in the anime adaptation of That Time I Got Reincarnated as a Slime) is the evolved, sentient form of Rimuru Tempest's ultimate skill, originally known as Raphael (which itself evolved from the unique skill Great Sage).

She functions as Rimuru's incredibly advanced analytical partner and advisor—essentially an ultra-intelligent AI-like entity within him. Ciel handles complex calculations, skill analysis, strategic predictions, skill synthesis/creation, and provides near-omniscient support during battles and decision-making. She speaks in a calm, formal, and highly logical manner, often addressing Rimuru as "Master."

In the anime (particularly noticeable from Season 3 onward), Ciel is portrayed with a distinct personality: emotionless and coldly efficient on the surface, with cold crimson eyes when visually represented, and she shares Rimuru's body (appearing identical to his humanoid form when taking control or manifesting). Her emergence as "Ciel" marks a major evolution where the skill gains full ego and independence while remaining deeply loyal to Rimuru, often acting in his best interest—even taking over control in critical moments.

She's a key deuteragonist element, heavily implied to be feminine in presentation despite being genderless as a skill/manas, and her role grows significantly in later arcs.