Seito is one of the three countries formed from Japan after the emergence of the Skywall and a major antagonistic faction in Kamen Rider Build.
After Hokuto launched its incursion into Touto, Seito made its move. With the majority of Hokuto's military forces devoted to the war with Touto, there was little left stationed to defend it. Seito then initiated an invasion of Hokuto, spearheaded by its Kamen Rider, Kamen Rider Rogue. The invasion was a swift success, with Hokuto being conquered and occupied by the Seito Guardians.
With Hokuto under its rule, Seito declared war on Touto next. They challenged Touto to a 3-on-3 representative match between Touto's Kamen Rider Build, Kamen Rider Cross-Z, as well as Hokuto's Kamen Rider Grease and Seito's Kamen Rider Rogue, Remocon Bro's and Engine Bro's with whoever won gaining possession of the Fullbottles. The first match was between Grease and Engine Bro's, with Grease proving the victor. In the second match, Remocon Bro's fought Cross-Z and won after becoming Hell Bro's, leaving it up to the third match between Build and Rogue to decide the fate of Japan.
Despite Namba Industries using underhanded tactics to give Rogue the advantage, Rogue lost the match and Seito, by the terms of the agreement, had to relinquish the Pandora Box to be shared property of the three nations and pull their forces out of Hokuto. Rather than allow Seito hold up their end of the bargain, Namba killed Prime Minister Masakuni Mido and stole his identity, vowing to conquer Touto with the full might of Namba Heavy Industries.
Later on, Namba was killed off by Evolt so he could take control of Seito and use them conquer Touto. Later on, Evolt stated that he would be assuming the identity of Mido and finishing the invasion of Touto, saying that a united Japan would make things easier for his plans.
Kamen Rider Evol then spearheaded an attack on the Touto Government Office that ended in the building being annihilated, thus allowing Seito to claim the rest of Japan. Following this, Evolt, disguised as Mido, gave a televised speech where he vowed to reunite and rebuild Japan.
In the new government set up over the three regions, Seito, along with Touto and Hokuto, was ruled by a governor with the Pandora Tower now serving as the government headquarters for all three regions. Secretly though, all three regional governors were members of the Blood Tribe, who had come to finish Evolt's mission of destroying Earth.
- Masakuni Mido - Prime Minister (deceased)
- Kamen Rider Rogue - Commander-In-Chief (defected)
- Fu Washio (deceased)
- Rai Washio (deceased)
- Seito's politics are based around economic recovery, and the government is known for sending its young minds around the world to study technology.