Erald Grimwald is an archduke of the Sorcerous Dynasty of Sarion, holding considerable political power as one of its highest-ranking nobles. He is a member of the Grimwald family, one of the thirteen royal families that govern Sarion, and his mother, Ellis Grimwald, is the leader of these families. Erald is the uncle of the young Emperor Elmesia El Ru Sarion, though he is notably much younger than his imperial niece. His brother, Sarion Grimwald, was killed in battle against the Chaos Dragon before the kingdom was founded and before Erald was born. Erald is also the father of Elyun Grimwald, an adventurer who is often called Ellen or Eren.

In his diplomatic role, Erald generally presents a calm and competent demeanor. However, this composed exterior belies an intense and often irrational protectiveness over his daughter, Elyun. His love for her is so overwhelming that it frequently overrides his judgment, leading him to act impulsively whenever he perceives she might be in danger, a trait that is well-known and often criticized by those around him, including his daughter's own adventuring party.

Erald's motivations are primarily driven by a desire to protect his family and secure the interests of his nation, the Sorcerous Dynasty of Sarion. His initial role in the story stems from a misunderstanding concerning his daughter's safety. Believing that Rimuru Tempest, the demon lord of the Jura Tempest Federation, had deceived or endangered Elyun, Erald traveled to Tempest intending to attack its leader. His daughter quickly intervened, clarified the situation, and physically stopped her father from acting on his false assumptions. Feeling partly responsible for Elyun's involvement in the events that led to Rimuru's evolution into a demon lord, Erald shifted his stance and became an ally. He collaborated with King Gazel Dwargo of Dwargon to create a cover story for the international community, falsely attributing the destruction of the Falmuth army to the Storm Dragon Veldora while crediting Rimuru with pacifying the dragon, a narrative designed to protect regional stability and Sarion's involvement. Subsequently, he was tasked by Emperor Elmesia with negotiating a peace agreement and fostering friendship between Sarion and the nation of monsters, the Jura Tempest Federation. Despite his personal reservations about allying with a monster nation, he followed his orders and participated in further diplomatic and economic discussions, including negotiations for a highway and a magic train route.

Erald maintains several important relationships. His bond with his daughter Elyun is the most defining, characterized by his extreme overprotectiveness. He shares a close friendship with King Gazel Dwargo; despite frequently bickering and trading insults, they have a strong mutual respect and share the common experience of being subject to the whims of Emperor Elmesia. His relationship with his niece, Emperor Elmesia, is complex; while he is a trusted and relied-upon figure in both public and private matters, Elmesia often treats him like a child and has been known to reprimand him for his diplomatic decisions, such as when he negotiated the construction of a highway at a disadvantage for Sarion's long-term rights. Over time, his relationship with Rimuru Tempest evolves from initial hostility to a cooperative and friendly alliance.

Erald is a skilled wizard with notable magical abilities. He is proficient enough to cast powerful, self-sustaining magic. One of his most distinctive skills is his ability to transfer his consciousness into a homunculus body, which he frequently uses when traveling outside of Sarion as a safety measure. His appearance is that of a gentleman with blonde hair styled to frame his face and distinctive slit-like green eyes that tend to widen markedly when he experiences intense emotions, such as worry about his daughter. He typically dresses in expensive and elaborate aristocratic attire befitting his high rank as an archduke.