In the anime Blood+, the character known as Carl, or by his alter ego Fantasma, is more formally named Karl Fei-Ong. He is a Chevalier, a powerful type of chiropteran, and a member of the clandestine organization Cinq Flèches, which serves his queen, Diva. His public identity is that of the refined and respected chairman of an all-girls' school in Vietnam called Lycee de Cinq Flèches. This composed facade, however, conceals a deeply disturbed and violent second personality known as the Phantom. Under this guise, he terrorizes his own school, leaving blue roses for girls before making them disappear in a desperate attempt to draw out Saya, his primary obsession.

Carl's background is one of manipulation and suffering, which fundamentally breaks his psyche. He was chosen by a fellow Chevalier, Amshel Goldsmith, to be a test subject for the Delta Project, a series of experiments that involved the repeated removal and replacement of his limbs. In 1917, he was brought before Diva, whom he asked to make him her Chevalier, but this transformation and the subsequent experiments left him severely mentally unbalanced. His traumatic past and Diva's inability to return the deep love he sought created an immense void in him. He became an outcast even among his fellow Chevaliers, feared by some like Amshel and disdained by others like James, with only Solomon Goldsmith showing him any consistent sympathy or loyalty.

Motivated by a singular and all-consuming obsession, Carl's primary goal is to see Saya in her berserker state, the feral and bloodthirsty form she takes when she fully awakens as a chiropteran. He is not driven by a desire for power or to serve Diva, but by a twisted need to reconnect with the Saya who fought and injured him in the past, a memory he seemingly cherishes. He considers Saya his hated enemy, his beloved soul mate, and his only reason for living, wishing to fight her again and even to die alongside her. This obsession leads him to betray his own organization when he feels his chance to confront her is being taken away.

In the story, Carl serves as a recurring and personal antagonist for Saya. He first appears as the mysterious Fantasma, confronting her at his school in Vietnam, where he is revealed to be the chairman. He leads an attack on the Red Shield, the organization that supports Saya, using experimental child chiropterans as soldiers and wounding key allies. His role is to embody the personal cost of Saya's past and the relentless nature of those who are obsessed with her power. His key relationships are defined by his dysfunction. He is devoted to Diva, but she is more of an object of unrequited love. He fears Amshel, who views him as a disposable tool, while Solomon acts as his only friend and eventual executioner. His entire existence revolves around Saya, whom he stalks and challenges repeatedly. Throughout the series, Carl’s development is a downward spiral of increasing desperation. After being physically wounded and losing his leg in a fight with Saya, he returns with new, even more dangerous abilities, including a prosthetic arm that can fire crystallized blood projectiles, a power he stole from the Schiff, another type of chiropteran. His mental state deteriorates further as he is sidelined by Cinq Flèches, culminating in a final, desperate attempt to kill Saya in London. His journey ends when Saya, realizing she has too much to live for, impales herself on her own sword to drive it through him, causing him to crystallize and shatter. A version of the character also appears in the Blood+ manga, where he is renamed Charles and has a significantly different, more childlike appearance and storyline, though many of his core traits, such as his obsession with Saya's berserk form, remain.