Karl Fei-Ong was introduced to Diva in 1917 by Amshel. When he met her, he requested that she turn him into a chevalier. Amshel later says he only chose Karl to use him as a guinea pig for their new Delta project. Karl's body was heavily modified by Amshel's experiments, including having the arm Saya destroyed in Vietnam in 1972 replaced with a spiked arm. His traumatic past, Diva's inability to give him the love he sought, and forced conversion seems to have left Karl severely mentally unbalanced. However, Solomon, the only one who could calm Karl's mania, believed Karl's problems stemmed from his being lonely and not seeing that Solomon was always there and watching over him.

Disguised as his alter ego "The Phantom", Karl kidnapped girls that looked like Saya in his desire to see her again to finish their fight. After finding her again, Karl became completely obsessed with killing her. He considered Saya his hated enemy, his beloved soul mate, and his only reason for living.

While in London, he stole the Corpse Corps from Amshel and went after Saya while she was at Gray's home. He planned to die together with her by drinking all of her blood. Solomon, who'd come after him at Amshel's request, cuts off Karl's arm freeing Saya's sword arm. Realizing she has too much to live for, Saya stabs him by impaling her own body. With her sword coated in her blood, Karl crystallizes and is destroyed. Solomon mourns his death and tries to explain Karl's actions.

Note: In the English Dub, his name is spelled Carl.