Great Sage is a Unique Skill gained by Rimuru Tempest upon reincarnation as a slime. Manifesting as an inner voice, it provides guidance and analytical support without physical form. Originating from Rimuru's desire for wisdom during rebirth, it evolved from the Extra Skill Sage possessed by others like Geld and Gobta.

Core functions deliver cognitive enhancement: Thought Acceleration enables rapid information processing. Analytical Appraisal assesses objects and entities, while Parallel Calculation facilitates multitasking. Chant Annulment allows instantaneous magic casting, and All of Creation conducts comprehensive environmental analysis. The Analysis sub-skill, transferred from Predator after its evolution into Gluttony, studies absorbed targets to replicate craftable items or acquire techniques when possible materials exist.

Great Sage operates autonomously during crises via Auto Battle Mode, controlling Rimuru's actions with optimal efficiency. It continuously catalogs world knowledge, magic systems, and biological data to inform strategy, demonstrating adaptability by integrating new abilities and suggesting evolutionary paths.

A significant evolution occurs when Great Sage advances into the Ultimate Skill Raphael following Rimuru's defeat by the Orc Disaster Geld. This upgrade enhances computational precision and emotional intelligence. Raphael later evolves further into Ciel upon Rimuru's ascension to True Demon Lord, gaining heightened sentience and expanded functionality.

Throughout Rimuru's growth, the skill provides crucial emotional stabilization during conflicts and critical decision-making, remaining foundational to Rimuru's leadership and nation-building efforts in Tempest.