Grucius is a skilled wolf beastman and a member of the Beastketeers, the elite guard serving under Demon Lord Carrion of the Beast Kingdom of Eurazania. As a member of the wolf tribe, his appearance features sharp eyes, pointed ears, and a tail, often clad in attire suited for swift, agile movement. His true beast form manifests as a werewolf, while his half-beast form maintains a humanoid structure, with gray fur covering his body and the head of a wolf. In humanoid guise, he presents as a robust, muscular man with ash-gray hair and dark-toned skin, typically wearing a sleeveless blue outfit and a white jacket. His age exceeds one hundred years, having undergone a transformation ceremony drinking Carrion's blood that changed his species, granting him a long lifespan and abilities similar to his king, though he remains the weakest among the Beast King's servants.

Grucius possesses a relaxed and open-minded personality, making it easy for others to form friendships with him. He is brave and loyal, with a tendency to speak his mind directly, and he values personal freedom, expressing a desire to expand his horizons by learning more about the world. He can also experience sadness or jealousy, particularly regarding people he cares for, but his underlying kindness and caring nature grow stronger over time. He holds deep respect for Demon Lord Carrion, whom he regards as a father figure. He also forges a strong, steadfast friendship with the human champion Youm Farmenas, a bond that forms following their initial battle, which ends in a stalemate and creates a lasting camaraderie.

Grucius first arrives in the nation of Tempest as a member of a diplomatic delegation from Eurazania, serving as a carriage driver. After establishing a non-aggression pact, he ends up stationed in Tempest as a long-term emissary, working with the city's security forces. It is during this time that he becomes close friends with Youm Farmenas and develops romantic feelings for Mjurran, a human mage. While initially unaware of Mjurran's secret ties to the demon lord Clayman, Grucius challenges her to a sparring match, loses due to a tactical wager, and subsequently becomes formally subordinate to Youm, though he remains loyal to Carrion should a disagreement arise.

When Clayman orchestrates an assault on Eurazania, Grucius allies with Tempest to repel the invasion and plays a role in freeing Mjurran from mental enslavement, aligning himself with Rimuru Tempest's objectives. Following the war against Tempest, a position opens for Knight Commander in the kingdom of Farmenas after the death of Forgan. When Youm ascends to the throne, he requests that Grucius fill this role, and Grucius accepts, taking on expanded military duties. Grucius evolves professionally and personally, navigating initial sorrow upon learning of Mjurran's pregnancy before embracing a fatherly role after the birth of her daughter, Mim, whom he openly claims as his own.

In combat, Grucius wields two knives with masterful skill, employing a flexible and creative fighting style that combines swift close-quarters slashing with long-range throwing attacks. His abilities include Far Sight, which enhances his perception, and Super-Recovery, a regenerative ability that allows him to heal. He also possesses a degree of resistance to physical attacks and excels at stealth and espionage, having been valued for these traits on missions to observe Tempest. Despite his long life and varied experience, he does not possess enough power to evolve on his own, instead relying on the ceremonial transformation granted by Carrion's blood, a trial with a ten percent survival rate that he successfully endured.