Randolph Ulster is a supporting character in the anime The Holy Grail of Eris. He is a senior officer in the royal military police, an unusual path for someone of his birth, since he is the eldest surviving son of the Richelieu ducal family. After his older brother Owen died of illness, Randolph was expected to inherit the ducal title, but he asked the current duke to allow him to keep the lesser Ulster family name instead. He chose to build his life on his own merits as an enforcer of the law rather than rely on his family's standing. He is a tall, imposing figure with short dark hair and blue eyes, and his strict bearing makes him appear stern and somewhat frightening at first glance.

His reputation among his colleagues is captured by his nickname, His Excellency the Grim Reaper. He is known for his keen instincts, his sharp eye for detail, and his refusal to compromise on his duties. He is earnest and serious by nature, and he is something of a workaholic, often heading to work even on his days off. At the same time, he can be surprisingly naive about matters outside his professional world, a trait that those closest to him notice and worry about. Beneath his intimidating exterior, he is a sincere and fundamentally just person who believes in upholding the law.

Randolph's past contains a deep personal tragedy. He was married to Lily Orlamunde, but she took her own life within roughly a year of their wedding. In the aftermath, social circles whispered that his cold demeanor had driven his delicate wife to despair. Randolph himself says nothing in his own defense; instead, he carries lasting regret over his failure to recognize the suffering Lily was enduring. Discovering the truth behind her death becomes one of his central motivations, and it is this pursuit that draws him into the main events of the story.

In the story, Randolph first acts as an obstacle to Constance Grail, the young noblewoman at the center of the plot. He catches her during a covert investigation connected to his late wife and is deeply suspicious of her intentions. Over time, however, he comes to believe in her sincerity. The two reach an arrangement in which he helps resolve problems facing her family in exchange for information, and their alliance deepens into a genuine partnership and eventually an engagement. Randolph becomes one of Constance's most important allies. He is also the first person to learn about the ghost of Scarlett Castiel, the executed noblewoman who accompanies Constance, and he helps investigate the case surrounding Scarlett's death and the conspiracy connected to it. His relationship with Scarlett is complicated, as she was once someone he regarded as a natural enemy, but the pursuit of the truth forces him to confront and revise that view of the past.

Among his other close relationships, he is especially close to Abigail O'Brian, the duchess of House O'Brian, who was born into a branch of the Richelieu family. Within the military police, his subordinate Kyle Hughes serves as his trusted right-hand man and often frets over his superior's guileless streak outside of work.

Over the course of the series, Randolph develops from a rigid, guilt-ridden officer into a more open and emotionally available person. Investigating his wife's death and the wider conspiracy behind it forces him to face his family's legacy, his own failures, and the walls he built around himself. His cooperation with Constance, built on mutual trust and sincerity, gives him a path toward resolving his guilt and moving forward.

In terms of abilities, Randolph is a highly capable investigator with sharp instincts and an exceptional eye for detail, able to see through disguises and deception that fool others. His rank in the military police grants him considerable authority and access, and he possesses the combat skill and physical capability expected of an officer in his position. His combination of investigative talent, official power, and personal conviction makes him a decisive figure in the story.