Fog Mother from the 1994 film Kamen Rider J is the central antagonist and the matriarch of the Fog, an alien race of nomadic, insect-like monsters. Her true nature is that of an ancient, parasitic lifeform, having first arrived on Earth approximately seventy million years ago. On that initial visit, her offspring consumed the dinosaurs, leading to their mass extinction, which established her species' predatory cycle of consuming a planet's dominant life forms. After a dormant period lasting millions of years, Fog Mother returns to Earth in the film's present day to repeat the process with the human race.

Physically, Fog Mother is a colossal entity, measuring seventy meters in height and weighing twelve thousand tons. She is not a single organic creature but rather an alien parasite fused with a formidable golden cybernetic mobile fortress, officially designated the Machine Beast Mothership Fog Mother. This mechanical body serves as both her armor and her weapon systems, equipped with a variety of artillery. These include Gatling guns, a laser cannon mounted in her right eye, circular saw blades that can be emitted from her left eye, and pincers and multiple insect-like legs that extend from her torso for close-quarters combat. Despite being a machine, she is capable of feeling pain when this armored body is damaged, indicating a direct neural link to the organic core within. True to her nature as a mother, the fortress body also functions as a mobile hatchery, containing the eggs for tens of thousands of her unborn children, who require a living sacrifice to hatch.

Fog Mother's primary motivation is purely instinctual and cyclical: the propagation and feeding of her species. Every millennium, she must give birth to a new brood, and this new generation requires the life force of the dominant species on whatever world she occupies. To this end, she commands her three eldest children, Garai, Zu, and Agito, to perform a ritualistic prayer and procure an ideal human sacrifice, a young girl named Kana, whose life is meant to awaken her dormant spawn. Her goal is the systematic consumption of all humanity, allowing her children to thrive and establish a new nest on Earth.

In the story, Fog Mother acts as the distant, commanding presence orchestrating the invasion from her hovering fortress. Her relationship with her children is defined by their roles as her agents and the continuation of her lineage. She refers to her eldest, Garai, as Prince, indicating a hierarchical family structure where her offspring serve as extensions of her will. She is utterly ruthless in her pursuit, showing no hesitation to destroy a nearby city to prepare for her children's awakening. Her development in the narrative is one of escalating direct involvement. After her children Garai, Agito, and Zu are defeated by Kamen Rider J, Fog Mother is forced to abandon her directive role and engage the hero personally in a desperate attempt to save her plan.

Her most notable abilities are tied to her massive mechanical form. In addition to her array of projectile and melee weapons, she can generate a mist that signals her arrival. Her primary method of attack against a giant opponent is to capture and attempt to digest them within her fortress body. In her final confrontation, after being severely damaged by Kamen Rider J's Jumbo Rider Kick, her body begins to self-detonate. However, she uses her final moments to reveal her true, vulnerable, insectoid core form, dragging J back inside her exploding ship in one last vengeful attempt to destroy him, which ultimately fails.