Gilbert Ross serves as the head forensics coroner at the Royal Investigative Service, a position that places him at the heart of criminal investigations in the anime B: The Beginning. His professional role belies his true nature, as he is ultimately revealed to be the main antagonist and a formidable adversary to the series' protagonists. Physically, Gilbert is distinguished by his short white hair, slicked back with two long strands on either side of his face, and his soft, bright blue eyes. His typical attire consists of dark turtleneck sweaters covered by a mid-length white lab coat, an outfit that complements his composed and professional demeanor.

On the surface, Gilbert appears to be a supportive colleague and a friend to the brilliant investigator Keith Flick. He initially comes across as a kind individual who expresses faith in Keith’s abilities and wishes for him to move past the traumatic death of his sister, Erika. However, this agreeable persona is a carefully constructed mask hiding a deeply disturbed and dangerous psyche. Gilbert suffers from dissociative personality disorder, a condition that allows the emergence of a ruthless and sadistic alternate personality, typically at night, during which he commits brutal murders. In this psychotic state, he is cunning and enjoys playing elaborate mind games with his victims and with Keith, taking pleasure in inflicting physical and psychological pain. Once his violent urges are satisfied, his mind resets to its normal state, effectively erasing the memory of his crimes and allowing him to return to his professional duties without any immediate recollection of his actions.

The core of Gilbert’s motivation lies in his complex and obsessive relationship with Keith Flick. He suffers from a profound inferiority complex, constantly lamenting his inability to surpass Keith, whom he mockingly refers to as Genie. This competitive envy drives his master plan, which is to create an intricate series of puzzles and crimes that would draw Keith deeper into his world of murder. His ultimate goal is to test Keith’s moral boundaries and force him into a situation where he must kill, thereby proving that the two are fundamentally the same. This obsession originated from a university conversation in which Keith stated he hated murder and could not understand it; Gilbert internalized this as a personal rejection and became determined to break Keith’s convictions.

Gilbert’s madness is intrinsically linked to Erika, Keith’s adopted sister. He was in love with her and, for a time, believed her presence could help him suppress his destructive urges. Upon discovering that Erika’s romantic feelings were for her adopted brother Keith, Gilbert’s obsession turned to homicidal rage; he kidnapped, tortured, and murdered her. His method was characteristically brutal, as he used his forensic knowledge to commit the perfect crime, slicing her into pieces and scattering her remains before using his underworld connections to frame another for the act. Despite being her killer, he continued to speak to Erika’s memory, demonstrating his inability to let go and his dependency on her image to maintain his fragile mental state.

In his role as the shadowy figure known as Regulus, Gilbert is the leader of a powerful black-ops organization called Market Maker. He uses this organization’s influence and resources to cover up crimes, manipulate evidence, and eliminate his targets. His history is steeped in destruction; he murdered his own father, who was the head of the Jaula Blanca Institute, and then orchestrated the massacre of nearly all the children and the lead researcher at the facility. He did this to destroy his father’s legacy and took the young, powerful Reggie named Minatsuki (also known as Laica) under his wing, manipulating him into becoming a key asset and killer for Market Maker. His murderous spree is extensive, as he is revealed to have killed 37 women who physically resembled Erika, in addition to his other numerous victims, storing their bodies in a hidden underground complex he built with Market Maker’s funds.

Throughout the story, Gilbert’s primary role is that of a puppeteer and a final adversary who forces Keith to confront his own past and morality. His key relationships are defined by manipulation and obsession: he is a false friend to Keith, a covetous murderer of Erika, and a master who exploits Minatsuki’s power for his own ends. His development culminates in a final confrontation at the ruins of the Jaula Blanca Institute. In a psychologically charged showdown, he taunts Keith with the truth of his sister’s death. He holds Keith’s colleague, Lily Hoshina, at gunpoint, deliberately creating the ultimatum he always desired: for Keith to kill him, thus shattering his moral code and proving his point. Keith ultimately shoots Gilbert to save Lily. In his final moments, Gilbert smiles, and it is theorized that his entire scheme was a suicide-by-cop plot; given that his multiple personalities prevented him from taking his own life, he manipulated Keith into doing it for him, finally ending his own madness. His influence extends beyond his death, as Keith continues to be haunted by Gilbert’s memory and arguments in the form of a hallucination during subsequent trials.

Gilbert’s notable abilities stem from his genius-level intellect, which matches Keith’s own. He is a master of forensics, which he perverts to destroy evidence and commit the perfect murders. His capacity for meticulous long-term planning, psychological manipulation, and his cold, calculated brutality make him a uniquely dangerous foe. He is a complete monster, not driven by supernatural forces, but by a deeply human and corrupted psyche that turned friendship, love, and admiration into tools for an elaborate, nihilistic game of murder.