He was portrayed by Kenji Ohtsuki.
Kaisei Mogami originally worked at the Touto Institute of Advanced Matter Physics where he discovered the Bugster Virus inside the Skywall. Theorizing that it came from a parallel world, Kaisei began working on a dimension bridging machine called the Enigma alongside Takumi Katsuragi.
When the Touto government did not approve his budget for the project Kaisei sought backing from the weapons manufacturing company known as Namba Heavy Industries Ltd.. While working at the company, Kaisei also developed a form of the Bugster Virus infused with Nebula Gas and created the Kaiser system.
However, Katsuragi discovered that Kaisei intended to use the Enigma device not just to create a bridge between the two dimensions but to merge them together and destroy them. Thus, Katsuragi quit working on the project and sabotaged the Kaiser system.
Completing the Enigma
2 years later, Kaisei put the finishing touches on the Enigma and began initiating the final stages of his plan to become ruler of both dimensions alongside his Ex-Aid counterpart. He set his Nebula Bugsters loose in order to lure out Build so he could claim the Doctor and Game Fullbottles from him. He was pursued by Cross-Z, but Kaisei managed to send him to the World of Build using his Enigma device.
His Ex-Aid counterpart meanwhile also set the Nebula Bugsters loose in order to empower his own Enigma machine. He then activates the machine and brings the Build Earth to the World of Ex-Aid.
Eventually, Kaisei as Left Kaiser merged with Ex-Aid Kaisei as Right Kaiser to become Bi-Kaiser as the final step of his plan to become ruler of both worlds. He was ultimately defeated by Kamen Riders Build and Ex-Aid.