This article is about a/an villain in Kamen Rider Build and Kamen Rider Ex-Aid.
For his Ex-Aid counterpart or the next "Left Kaiser", see Kaisei Mogami (World of Ex-Aid) and Fu Washio.

―Transformation phrase as Kaiser.[src]

―Transformation phrase as Bikaiser.[src]

"I am the emperor. Kamen Riders are no more than dust."
―Kaisei taunts Build, Ex-Aid and other Legend Riders.[src]

Kaisei Mogami (最上魁星 Mogami Kaisei) was a former researcher of the Touto Institute of Advanced Matter Physics. Using the Nebulasteam Gun, Kaiser become Kaiser (カイザー Kaizā), also referred as Left Kaiser (レフトカイザー Refuto Kaizā).

Character History

Kaisei Mogami was a researcher of Touto Institute of Advanced Matter Physics, specializing in the study of parallel worlds. He discovered the Bugster Virus in the Sky Wall, and theorized that it came from another world. That theory would eventually lead to him developing a cross-dimensional machine called Enigma alongside none other than Takumi Katsuragi. The government did not approve of his budget and Mogami quit his job, and instead went to work for Namba Heavy Industries Ltd.. During his time there, he managed infuse the Bugster Virus with the Nebula Gas and devised the Kaiser system, but actually building it required a more potent sample of the Bugster Virus, so Namba allowed him to work on Enigma, while Takumi's job at Faust provided him to means to go to Ex-Aid's world as Kamen Rider Build in order to collect Bugster virus samples. [1]. Sooner or later, Takumi Katsuragi would discover Kaisei Mogami's true intentions. Enigma was not built to solely transit between Build and Ex-Aid's worlds, but rather to combine and inevitably destroy them. Mogami knew this fact, yet he was hell bent on doing so since his survival was guaranteed. Realizing his true goals, Katsuragi sabotaged the Kaizer system and abandoned all further development of Enigma.

Two years later, Mogami would unleash Enigma onto Build's World. Seeing Build use the Ex-Aid-derived Doctor and Game Fullbottles to wipe out his Nebula Bugsters, Mogami became the Kaiser and entered the fray, attacking and instantly forcing Build out of his transformation as he confiscated the aforementioned Fullbottles. The Kaiser was, however, pursued by Kamen Rider Cross-Z, who found him overlooking the Enigma device by the Sky Wall, which would allow him access to the "other side". Besting Cross-Z, Mogami gave him a demonstration of Enigma's transdimensional properties as he threw him into the portal as it activated, depositing him to Ex-Aid's world, though not before Cross-Z struck the Doctor and Game Fullbottles from his person. The Kaiser attempted to recover the Fullbottles but was prevented by the Kaizoku Hassyar fired by Build KaizokuRessya. When he attempted to engage Build again, the Kaiser developed a fault in his system, forcing him to take his leave.

Sento found his lab and confronted the mad scientist, who after destroying both worlds would assimilate with his Ex-Aid counterpart and become immortal, gaining dominance over all parallel worlds. Mogami revealed himself to be a hologram and said his plan was counting down. Kamen Rider Heisei Generations FINAL: Build & Ex-Aid with Legend Riders


Facing a new model Bikaiser, Sento Kiryu initially believed that Kaisei Mogami had returned, bewildered as to how he could have survived. When he defeated the Bikaiser, however, he found the true identity to be an innocent test subject whom had been exposed to Nebula Gas. Infact, the late Mogami's Kaiser System had been updated by Nariaki Utsumi on behalf of Blood Stalk and Namba Heavy Industries Ltd., and the prototype's battle with Build allowed him to finalize the new Hell Bros. Next Chapter

Sento was again reminded of the unification of the two Kaisei Mogamis into Bikaiser when he witnessed Fu Washio transform into Hell Bros. The Counterattack Hero


He is a violent, evil scientist bent on annihilating all humans, though in comparison to his World of Ex-Aid counterpart, he is the calmer and more sadistic of the two.


Nebula Hell Bro's
Gearless Bi-Kaiser

Nebula Hell Bro's

Nebula Hell Bro's (ネビュラヘルブロス Nebyura Heru Burosu) is the basic body of the Kaiser with no Gears attached. Though typically bypassed altogether in the transformation to Kaiser, it is briefly seen during the transformation to the combined Bikaiser. Kamen Rider Heisei Generations FINAL: Build & Ex-Aid with Legend Riders

Left Kaiser


"Gear Remocon! Funky! (Rhythmic beeping accompanied by steady rock music) Remote Control Gear!"
―Transformation announcement[src]


  • Height: 202.0 cm.[2]
  • Weight: 103.5 kg.[2]

Ability Perimeters:

  • Punching Power: 20.3 t.[2]
  • Kicking Power: 24.8 t.[2]
  • Maximum Jump Height: 32.4 m.[2]
  • Maximum Running Speed: 100 m. per 2.8 sec.[2]

Kaiser (カイザー Kaizā), also named Left Kaiser (レフトカイザー Refuto Kaizā), is the blue Kaiser, accessed by inserting the blue Gear Remocon into the Nebula Steam Gun.

Kaiser's finisher is the Funky Drive (ファンキードライブ Fankī Doraibu): Kaiser channels the power of a Gear and performs a powerful blast attack.

  • Gear Remocon: Kaiser fires a blue large gear energy shot at the enemy.

Appearances: Heisei Generations FINAL: Build & Ex-Aid with Legend Riders, Build Episode 23 (archive footage only)



"Gear Engine! Gear Remocon! Funky Match! Fever! (Rhythmic beeping, engine revving) Perfect!"
―Transformation announcement[src]


  • Height: 202.0 cm.[2]
  • Weight: 108.0 kg.[2]

Ability Perimeters:

  • Punching Power: 40.6 t.[2]
  • Kicking Power: 49.6 t.[2]
  • Maximum Jump Height: 64.8 m.[2]
  • Maximum Running Speed: 100 m. per 1.4 sec.[2]

Bikaiser (バイカイザー Baikaizā) is the fusion of Kaiser and Kaiser Reverse, accessed when both Kaisers transform together.

Appearances: Heisei Generations FINAL: Build & Ex-Aid with Legend Riders, Build Episode 27 (archive footage only)



Behind the Scenes


Kaisei Mogami is played by Kenji Ohtsuki (大槻 ケンヂ Ōtsuki Kenji).


In the script of Kamen Rider Heisei Generations FINAL: Build & Ex-Aid with Legend Riders, Kaisei Mogami's form was dubbed the L Kaiser (エルカイザー Eru Kaizā, Left Kaiser), equal to the R Kaiser.[3]


  • According the profile that Gentoku Himuro given to Sento, Kaisei Mogami is 40 years old. However his actor Kenji Ohtsuki is 51 years old at that time.
  • His origin is similar to Two-Face from DC Comics.
  • His appearance is similar to both Two-Face and Cyborg, who also is from DC Comics.



Sento Kiryu (Takumi Katsuragi) - Ryuga Banjo - Kazumi Sawatari - Gentoku Himuro - Evolto - Nariaki Utsumi - Shinobu Katsuragi - Kengo Ino - Killbus - Keiji Uraga
Transformation Gear
Fullbottles - Build Driver - Transteam Gun - (Great) Cross-Z Dragon - Sclashjellies - Sclash Driver - Hazard Trigger - Nebulasteam Gun - Cross-Z Magma Knuckle - Evol-Driver - Evol-Trigger - Grease Blizzard Knuckle - Killbuspider - Grease Perfect Kingdom
Drill Crusher - Hawk Gatlinger - 4Koma Ninpoutou - Kaizoku Hassyar - Fullbottle Buster - Steam Blade - Beat Closer - Twin Breaker - Muscle Glove - Solstall Wings - Giant Scratcher - Space Ride Arm - Spine Knuckle - Long Range Cleaner - Trash Converter - Gold Lio Gauntlet - Multi Deluge Gun
Pandora Box - Halfbodies - Fullbottle Holder - Build Phone
Vortex Finish - Hazard Level - Project Build - Sky Wall - Nebula Gas
nascita: Misora Isurugi - Soichi Isurugi - Sawa Takigawa - Kazumi Sawatari - Gentoku Himuro
Prime Ministers: Taizan Himuro (Touto) - Yoshiko Tajimi (Hokuto) - Masakuni Mido (Seito)
Namba Heavy Industries Ltd.: Juzaburo Namba - Nariaki Utsumi - Haruhiko Wanibuchi - Guardian
Hell Bro's: Fu Washio - Rai Washio - Yoshikazu Takahashi - Norio Matsui
Aliens: Vernage
Downfall: Keiji Uraga - Simon Marcus
Gentoku Himuro - Evolto - Nariaki Utsumi - Takumi Katsuragi - Shingo Kuwata - Eita Kawai - Yoshikazu Takahashi - Norio Matsui
Touto: Needle Smash - Strong Smash - Burn Smash - Flying Smash - Mirage Smash - Square Smash - Press Smash - Ice Smash - Stretch Smash
Hokuto: Castle Hard Smash - Stag Hard Smash - Owl Hard Smash
Kaiser: Kaisei Mogami (World of Ex-Aid)
Kengo Ino - Ryoka Saiga - Mitsuomi Gobara - Evolto - Killbus
Community content is available under CC-BY-SA unless otherwise noted.