Born on May 17, 1986, in London to shaman parents Liam and Jean Diethel, Lyserg Diethel received training as a dowser from his father starting at age four. This culminated at age six in a test where he located a key to free the ancestral poppy spirit Morphin (also called Morphea or Chloe), who became his guardian ghost. Returning home successful, Lyserg discovered Hao Asakura standing over his parents' burning corpses; they had been killed for refusing to join Hao. Lyserg confronted Hao but was easily defeated due to inexperience, managing only to grab his father's pendulum before losing consciousness. Rescued by firefighters, he lived with his maternal grandfather until the latter's death. Lyserg was then taken in by the human trafficker Oknox, who later killed Lyserg's friend Wat Hudson and attempted to murder Lyserg. Lyserg survived the shooting, while Oknox was killed by Hao. This fueled Lyserg's lifelong quest for vengeance against Hao.

Qualifying for the Shaman Fight under Patch Officiant Bron, Lyserg sought allies to locate Patch Village. His dowsing pendulum led him to Yoh Asakura's group. Initially mistaken for a girl by Ryunosuke Umemiya due to his androgynous appearance—characterized by light green hair and eyes—Lyserg proved his gender demonstratively. He showcased his combat abilities by incapacitating Horohoro and Tao Ren using his Crystal Pendulum Over Soul, channeling Morphin into a high-speed wire weapon. A subsequent confrontation with Yoh ended with Lyserg joining the team after recognizing Hao's likeness in Yoh's face triggered traumatic memories.

During the journey, Lyserg's vengeful impulses surfaced when encountering Hao's followers, leading to a confrontation where he was bitten and possessed by vampire Boris Tepes Dracula's spirit. After Yoh's group freed him, the X-Laws intervened to execute Boris. Impressed by their power and shared goal of destroying Hao, Lyserg abandoned Yoh's team to join the X-Laws under Marco Lasso. He adopted their distinctive white uniform—featuring an open skirt, shorts, and boots—and received the mechanical Archangel spirit Zelel, modeled after a Lotus Elise sports car. Lyserg became part of Team "X-I" with Marco and Iron Maiden Jeanne. However, he conflicted with the X-Laws' extremist methods, particularly their willingness to sacrifice members like Team "X-III" against Hao. Witnessing their slaughter by Hao and Marco's subsequent defeat by traitor Luchist deepened his disillusionment.

Chosen as one of the Five Elemental Warriors by Sati Saigan, Lyserg underwent training in Hell, emerging with enhanced furyoku (170,000). He discarded his X-Laws uniform for a black cloak with green accents and fused Morphin and Zelel into the Mastema Dolkeem Over Soul, an armored form designed to contain his hatred rather than fight Hao. During the final battle, Hao granted him the Spirit of Fire, enabling attacks like "Cremation" and "Magma Upper." Post-tournament, Lyserg reconciled with Yoh's group. Years later, as depicted in "Funbari no Uta," he appeared taller at age 21, wearing a detective trench coat, having pursued his dream to become a detective like his father.

Lyserg's personality balanced a kind heart with anti-hero traits, driven by obsessive hatred for Hao that caused volatile reactions to his name or presence. He admired his detective father, read mystery novels, and valued rationality, noted as the cleverest of the Five Elemental Warriors. His appearance evolved from a Sherlock Holmes-inspired green cloak to X-Laws regalia and finally darker, practical attire. Throughout his arc, Yoh's philosophy prevented him from fully embracing the X-Laws' ruthlessness, ultimately guiding him back from vengeance toward his original ideals.