Grigori is a character from the anime and light novel series That Time I Got Reincarnated as a Slime. He is known as one of the Three Martial Sages and is also a member of the Ten Great Saints, an elite group of powerful holy knights serving the Holy Empire. Grigori is also referred to by the epithet Giant Rock, a name that reflects his most defining physical trait: an incredibly tough and sturdy body that is said to be as unmovable as stone. In keeping with his formidable physique, his weapon of choice is a large halberd.

Personality wise, Grigori is portrayed as hot-blooded, fiery, and direct, preferring action and plain speaking over formal or polite behavior. This straightforward and bold nature is clearly demonstrated during an important meeting with Hinata Sakaguchi, the head of the temple knights. When Hinata announces her decision to pursue a policy of non-aggression towards Rimuru Tempest and the nation of Tempest, Grigori openly and fiercely opposes her, showing he is not afraid to challenge authority or speak his mind.

Grigoris role in the story is primarily tied to a significant military mission. He is dispatched to the Kingdom of Falmuth as part of a force intended to subdue Youm Farmenas and the former King Edmaris Falmuth. It is during this expedition that he confronts several key allies of Rimuru: the kijin swordsman Hakurou, the star wolf Ranga, and the lizardman fighter Gabiru. This confrontation ends in a decisive defeat for Grigori. Ranga proves to be overwhelmingly powerful, swiftly neutralizing all of Grigoris allies and leaving Grigori himself badly wounded and helpless.

This catastrophic failure serves as a major turning point for him. Unable to return to the Holy Church or the Holy Empire in disgrace, Grigori finds his path irrevocably altered. Along with his fellow Martial Sage, Saare, he is taken in by the sorcerer Razen. Instead of serving as a frontline holy knight, Grigori is subsequently assigned to work in the shadows, providing support for the future kingdom of Farmenas. His development is thus a shift from a proud and prominent warrior to a more covert operative working behind the scenes. His key relationships are with his fellow Martial Sages, Saare and Glenda Attley, with whom he often works, and later with Razen, who becomes his patron after his fall from grace.

In terms of notable abilities, Grigoris primary power is his superhuman endurance and the absolute toughness of his body, which allows him to withstand immense punishment in battle. This defensive capability, described as the unmovable rock, is what he is most famous for among the Ten Great Saints. He complements this defensive strength with his offensive skill in wielding a halberd in combat.