Kougane is the manifestation of an artificial Golden Fruit created by an Over Lord in the past. His awakening eventually accelerated the race's extinction as they fought to the death. Lapis, a young Over Lord sealed him at the cost of his own. Nonetheless, his curse on the Femushinmu remained, leaving them endangered. Ryoma Sengoku discovered his seal and unintentionally released him, resulting in his own death. Kougane's existence causes the Armored Riders to lust after each other for the "Golden Fruit". Slowly, the Armored Riders start to kill each other off as Kougane masquerades as Yuya to get to Kouta. The deaths of the fallen Armored Riders strengthens Kougane's "Golden Ringo" Lockseed. When it is revealed that he is not really Yuya, Kougane transforms into Armored Rider Mars and defeats Kouta. Corrupting Gaim into Gaim Yami, the two go after Lapis. However, Ryugen, Baron, and Marika interfere, with Lapis disrupting the mind control on Kouta. Kouta transforms into Gaim Kachidoki Arms and chases Mars in a fight that ultimately allows most of the deceased Riders, whose hearts were not fully corrupted like Sigurd and Duke, to return to aid Kouta. Mars emerges as a flame horse and is defeated by the Armored Riders as the world returns back to normal, also resulting in Lapis' disappearance.
Post-Helheim Invasion
Kougane survives his encounter in the movie and returns after seven months has passed since the Helheim plants and Inves were removed from Earth. Taking the form of a locust, he possesses a teenage girl as leverage in case if he's at a disadvantage. Using the Black Ringo Lockseed, he is able to assume the form of Armored Rider Jam, a substantially weaker form than Armored Rider Mars, due to the energy in the Golden Ringo Lockseed being completely drained after his original defeat. He seeks out the other Armored Riders for forcing him into disgrace only to realize that Lapis had altered everyone's memories to make it appear as if it were a dream.
Taking on Kougane first is Jonouchi, honoring the late Ryoji Hase by transforming into Kurokage. However, Kougane easily defeats Jonouchi without any effort. Next in line to fight against Kougane is Mitsuzane, who puts up a fight despite being at a disadvantage with Jam and Grasshopper Monster against him. Kougane reveals that he's using a human body as his vessel, causing Mitsuzane to cancel his transformation. However, Kouta appears, revealing that he left one loose end and that's Kougane. Kouta destroys the Grasshopper Monster in his wake and forces Kougane out of the girl's body. Kougane is killed by Mitsuzane and Kouta's Rider Kicks as they pushed his own Darkness Au Lait back at him. At the end of the series, the girl who was possessed by Kougane earlier is teaching two young boys, who were the two kids Kouta and Kaito had helped out a long time ago, about the sacred tree and dancing.