Carl Feion, whose name is also presented as Karl Fei-Ong, is a secondary antagonist and one of the Chevaliers serving Diva. Originating from Vietnam, he was once a medical student before being selected by Amshel Goldsmith around 1917 to serve as a test subject for the Delta Project, an experiment to determine if people of Mongoloid descent could become Chevaliers. It was through his friend Solomon Goldsmith that he was introduced to Diva, whom he then requested to transform him into a Chevalier, a decision that would bind him to her for nearly a century. Before the main events of the story, Carl had a previous confrontation with Saya during the Christmas Bombing in Vietnam in 1972, a fight in which she destroyed his right arm.

Carl presents a dual nature that borders on a dissociative identity disorder. On the surface, he maintains the calm, mature, and polite persona of the chairman of Lycee de Cinq Fleches, a prestigious all-girls school in Vietnam where he is respected by students and faculty. However, this civility masks his alternate identity as the Phantom, a violent, obsessive, and sadistic killer. His defining characteristic is a profound and manic obsession, directed primarily at Saya and secondarily at Diva. He does not simply wish to defeat Saya; he sees in her a kindred loneliness that he finds deeply compelling, leading him to desire a murder-suicide with her as a way for them to finally belong together. This instability frequently causes him to disobey direct orders from Amshel to pursue his own violent agendas, to the point where his fellow Chevaliers, particularly James and Amshel, regard him as reckless and disposable.

Within the organization Cinq Fleches, Carl is the youngest of Diva's five knights and is often the outcast. His most significant relationship is with Solomon Goldsmith, who was instrumental in his transformation. Solomon feels great sympathy and guilt for Carl's fate, believing his insanity stems from loneliness and blindness to Solomon's own constant, watchful care. In contrast, Carl fears Amshel, the mastermind behind the painful experiments that modified his body. While he initially held a fondness for Diva, her rejection of his unstable behavior deepens his psychosis and redirects his obsession entirely toward Saya, whom he sees as his ultimate enemy and soulmate.

Carls role in the story begins with his introduction as the Phantom, who kidnaps schoolgirls resembling Saya in a twisted attempt to lure her into a final confrontation. He engages Saya and the Red Shield multiple times, including an attack on their headquarters where he personally escorts Diva and severely wounds a Red Shield operative, Joel, with his spiked arm. His development follows a clear trajectory of mental deterioration. Originally a willing but manipulated subject, his repeated failures against Saya, combined with the loss of his limbs and Diva's growing indifference, strip away any remaining sanity, leaving only a raw, suicidal desire to destroy himself and Saya together.

Carls notable abilities are those of a Chevalier, including enhanced strength, speed, and regenerative healing. His body was heavily modified by Amshels experiments after he lost his arm to Saya. His original power allowed him to form a blade from his hands, but his replacement right arm was transformed into a terrifying weapon capable of firing projectiles made from crystallized blood, similar to the abilities of the Schiff. He also possesses the standard Chevalier traits of flight, manifesting wings from under his arms, and an advanced sensory ability to detect other Chiropterans. His final fight with Saya takes place in London, where his plan to drain her blood and die alongside her is foiled by Solomon, who cuts off his modified arm, leaving Saya free to impale him through her own body, ending his life.