A 26-year-old male. A self-proclaimed genius physicist who was kidnapped by a mysterious organization and subjected to human experimentation, resulting in amnesia. The only fragment of memory he retains is the image of a bat-like man who supervised the experiments. Found wandering the streets with no memory by Isurugi Soichi, he now lives with him while hunting Smash—monsters appearing in Touto, a region of Japan divided by the Sky Wall—fighting daily to reclaim his memories. His current name was given to him by Soichi, as he claims not to remember his original name. He is Kamen Rider Build.

A 23-year-old male. Kamen Rider Cross-Z. Simple-minded and clumsy, but his sharp instincts and sheer grit are second to none. Originally a professional fighter, he was stripped of his license and given a suspended sentence after becoming involved in "match-fixing" to pay for his girlfriend Kasumi's surgery. Later, he was framed for the murder of a Touto Institute of Advanced Matter Physics researcher and became a prisoner. After escaping, he is now being pursued by the Touto government's special forces led by Himuro Gentoku.

A 29-year-old male. Kamen Rider Grease. A soldier from Hokuto. He was sent to Touto under orders from Hokuto Prime Minister Tajimi Yoshiko to attack civilians and Riders. Although he has personal grievances, he maintains that as a soldier, he cannot harbor private emotions and transforms into Grease to raid Touto. His true occupation is a farm owner, and he is called "Boss" rather than by name by the Hokuto Three Crows and other farm workers. While he is cool-headed when not transformed, he occasionally shows a clumsy side in front of Isurugi Misora.

A 35-year-old male. Kamen Rider Rogue. The Touto Prime Minister's aide and Director of the Touto Institute of Advanced Matter Physics. A brilliant and highly ambitious man with immense pride. As the Director of the institute, he hired Kiryu Sento after he scored perfectly on a test and ordered him to research the Pandora Box discovered on Mars.