Kouji Segawa (瀬川 耕司, Segawa Kōji) is an environmentalist reporter who was investigating the pollution by the lake. He was killed while attempting to protect his young friend, Kana and was thrown over the cliff by the Fog Mother's child Agito as Kana is taken by the insectoid Zu. However, Kouji was resurrected by the Earth Spirits as a J-Powered Warrior, Kamen Rider J, acting on their will to fight the Fog with their emissary Berry as his guide. With Fog Mother sensing Kouji's presence, Agito attempts to finish the job in his true form, only to be killed by J. Hurrying to Kana, Kouji enters their domain and faces Zu as she assumes her true form. Facing her, J manages to mortally wound Zu as she grabs him, taking to before they crash into the fortress which is Fog Mother.
After she dies, Kouji learns of Fog's intention of feeding humanity to Fog Mother's brood as Garai completes the ritual and sends Kana to the hatching chamber while manhandling Kouji. Berry intervenes to free Kana from Fog's spell before being struck down by Garai as Kana is sent down to her death while Fog Mother begins attacking a nearby city to prepare for her children's awakening. Assuming his Rider form, J fights Garai in his true form, Cobra Man, in a heated battle. After killing Garai, J is being digested by Fog Mother as he attempts to save Kana. However, absorbing the life energy around him, J kills off Fog's newly born spawn as he escapes the bowels of Fog Mother and assumes Jumbo Formation. After ripping Kana out of Fog Mother, J battles the monster before killing her as she attempted to take him down with her. Escaping Fog Mother's destruction, Kouji takes Kana to a serene environment with Berry watching from a distance.
The J-Spirit (Jスピリット Jei Supiritto?) is Kamen Rider J's transformation belt.
The J-Crosser is a Suzuki off-road bike that Kamen Rider J rides on. It has the ability to reach a speeds up to 1300 km/h (807 mph). Kamen Rider J also can use the J-Crosser to execute the J Striker attack.
- Dex Stewart - Kotaro Minami's American counterpart in Saban's Masked Rider.