Felzeb is a symbiotic entity dwelling within Belzeb, commander of the Jaku Empire's invasion forces. Emerging when Belzeb's chest cavity opens, Felzeb appears as a small, fairy-like being. This entity serves as Belzeb's close advisor and communicator, frequently relaying strategic insights and information from the Jaku Empire command during their campaign against Earth and the Earth Defense Class.

Throughout the television series, Felzeb observes battles involving Raijin-Oh and later God Raijin-Oh directly from within Belzeb. Felzeb continues this role in the four-episode OVA sequel, participating in its intensified conflicts. During one notable OVA sequence, Felzeb actively engages in combat against the heroes inside a human body during a "Fantastic Voyage" scenario.

Felzeb is present for the Jaku Empire's final assault in the series finale, including the deployment of the empowered Great Jaaku Satan mecha and the subsequent psychic battle Belzeb initiates against the Raijin-Oh pilots. After Belzeb accepts defeat and ceases hostilities following the psychic attack's failure, Felzeb departs Earth alongside Belzeb and the retreating remnants of the Jaku Empire forces. No additional information exists regarding Felzeb's origins, motivations, or ultimate fate separate from Belzeb beyond the television series and its direct OVA sequel.