Keith Eigrid acts as a crucial aide to Claude Jeanne Ellmeyer, the demon king and first prince. His loyalty originates from childhood when Claude saved him from drowning, an event that exposed Claude's demonic powers and caused his disinheritance and exile. Keith followed Claude into banishment, becoming one of his few trusted humans. He manages Claude’s household administration with quiet composure, overseeing finances, budgets, tax collection, and even preparing Claude’s tea.

In the otome game *Regalia of Saints, Demons and Maidens*, Keith betrays Claude by secretly selling demons to humans—an act that triggers Claude’s transformation into a dragon and death. This betrayal, however, is coerced: Keith trades demons under blackmail from a human lord who threatens eviction due to forged land deeds for Claude’s castle. Keith tracks buyers to pressure humane treatment, living in shame while prioritizing his lord’s security.

Aileen Lauren d'Autriche disrupts this fate, uncovering the blackmail and enabling Keith to confess to Claude before irreversible harm. Her intervention redeems Keith, resolves the land deed forgery, and lifts his burden of secrecy. Post-redemption, he remains a trusted advisor in Claude’s inner circle, strategically balancing the volatile demon aide Beelzebuth.

Keith has brown hair, blue eyes, and wears glasses, typically dressed in formal attire: vest, shirt, trousers, and bowtie. His relationships center on unwavering devotion to Claude, growing respect for Aileen after her aid, and a professional dynamic with Beelzebuth. Over time, he evolves from isolated guilt to operating openly alongside Claude and Aileen.