is a Japanese tokusatsu series that serves as the 28th installment in the Kamen Rider franchise and the 19th entry in the Heisei era.

Main characters Sento Kiryu is the main protagonist of the series. He is a genius physicist with amnesia, with his only memory being Night Rogue performing experiments on him, and was found by Sōichi Isurugi and offered shelter at Nascita. It is eventually revealed that his true identity was the demon scientist Takumi Katsuragi, whose memory was erased after a failed assassination attempt on Evolto and faced swapped with . After learning of his true identity, there are times where Sento's and Takumi's personalities swap places, but the Takumi personality eventually chooses to disappear so Sento can defeat Evolto.

After the final battle, Sento is teleported to the "new world", which is actually the main Kamen Rider universe. Here, he is sometimes mistaken for this world's version of Tarō Satō, whom he becomes a fan of as revealed in Kamen Rider Zi-O.

Sento transforms into Kamen Rider Build by utilizing the belt, which combines the essence contained within a pair of to access several forms. Certain combinations work better than others, with optimal combinations known as a and mismatches known as a . His default Best Match is via the and Fullbottles. While transformed, he wields the , which can switch between and . His personal vehicle is the motorcycle, which is summoned by using the Fullbottle in conjunction with the . Alongside his numerous Best Matches and Trial Forms, Sento develops several power-up forms, including: : An upgraded version of Rabbit Tank Form accessed from the drink can-like device that grants the ability to summon any of Sento's preexisting forms' weapons. In this form, he is equipped with a pair of forearm-mounted . : Sento's super form accessed by combining the Build Driver and the adapter while using any Best Match combination in conjunction with the former that allows him to increase the capabilities of the corresponding form into a at the risk of losing control. He later creates the to stabilize the Hazard Trigger and evolve this form into one of two stronger versions. In both of this form's evolutions, he wields the , which can switch between and . : A red-colored evolution of Hazard Form accessed from the Full Full Rabbit Tank Bottle's that grants superhuman speed. : A blue-colored evolution of Hazard Form accessed from the Full Full Rabbit Tank Bottle's that grants superhuman strength. : Sento's final form accessed from the Fullbottle that grants the combined powers of all 60 original Fullbottles. : A fusion form between Kamen Riders Build and Cross-Z accessed from the device that grants the use of the support robot and the energy construct for combat assistance. This form first appears in the film Kamen Rider Build the Movie: Be the One.

Sento Kiryu is portrayed by .

Ryuga Banjo is a 23-year-old former professional fighter from Yokohama. Prior to his birth, a portion of Evolto's essence fused with him in a failed attempt to possess his mother . As a result, Ryuga was born after only two months of gestation, and spent much of his childhood being tested on by Namba Industries. After Takumi Katsuragi learned of Ryuga's true nature, he attempted to lure him into his apartment and kill him and Evolto, but the plan backfired when Evolto erased Takumi's memories and killed Tarō Satō, with Ryuga being framed for the murder. After escaping prison, he was taken in by Sento Kiryu. His girlfriend was turned into a Smash and killed in front of him, with the resulting essence purified into the Fullbottle, which was given to him.

After the final battle, Ryuga is teleported to the "new world" where he encounters his alternate self off-screen before finally reuniting with Sento.

Ryuga receives the robotic adapter from Sento as a companion. He is also given a copy of the Build Driver, which he can combine with the Cross-Z Dragon while utilizing the Dragon Fullbottle in conjunction with the latter to transform into . While transformed, he wields the sword. Unlike Build, Cross-Z cannot mix and match Fullbottles, as the Cross-Z Dragon only has one slot, but he compensates by being physically stronger. Over the course of the series, Ryuga acquires several evolutions of his Rider form, including: : An upgrade form accessed from the Dragon Sclashjelly in conjunction with the Sclash Driver that grants similar capabilities as Kamen Rider Grease. While transformed, Ryuga wields the Twin Breaker. : Ryuga's final form accessed by combining the Build Driver and the pyrokinetic while using the Fullbottle, which is created from the Dragon Sclashjelly, in conjunction with the latter that grants a pair of . If necessary, he can wield the Cross-Z Magma Knuckle in battle. : Ryuga's super form accessed by combining the Build Driver and an upgraded version of the Cross-Z Dragon called the while using the Evolbottle, which is created from the Dragon Evolbottle, in conjunction with the latter that increases the capabilities of his default form. : A fusion form between Kamen Riders Cross-Z and Evol accessed from the Fullbottle that allows the former to use the latter's powers. This form appears exclusively in the V-Cinema Build New World: Kamen Rider Cross-Z.

Ryuga Banjo is portrayed by .

Misora Isurugi is Sōichi's 19-year-old daughter, who works as Sento's assistant in purifying Fullbottles. She gained her powers after being exposed to the and spending seven years in a coma. As well, she played a role in Project Build, as Evolto used Sōichi's memories of her drawings of organic and inorganic-based objects to influence Shinobu into developing the Best Match system. Faust kidnapped her for experimentation, but Sōichi rescued her a year prior to the series' events. She remained hidden in Nascita while using her online idol identity to gather intel and money from her fanbase.

Misora possesses the ability to purify and enhance Nebula Gas, which made her a target of Faust. Initially, using the ability put her to sleep for a week, but Sento later developed a machine to mitigate this. During the invasion by Seito, the western part of Japan, Misora's Martian bracelet is revealed to house the spirit of the Martian Vernage and is the source of her power.

The new world version of Misora regains the memories of her alternate self when the Pandora Box is reconstructed during Killbus' invasion and accepts Kazumi's proposal during the events of Downfall's attack.

Misora Isurugi is portrayed by . As a child, Misora is portrayed by .

Recurring characters Sawa Takigawa is a 27-year-old freelance journalist who assists Sento after learning of his true identity as Kamen Rider Build. She is later revealed to be a spy for Namba Heavy Industries, using her position to investigate the cover-up of an industrial accident she believes killed her father. She is actually a brainwashed Namba agent, with her motivating drive about her father being a fabrication. She was supposed to let herself die after failing Namba's mission, but the familial bonds she made with Sento and the others gave her a reason to live and influenced her decision to cut ties with Namba.

The new world version of Sawa regains the memories of her alternate self when the Pandora Box is reconstructed during Killbus' invasion and later enters a relationship with Gentoku.

Sawa Takigawa is portrayed by .

Nascita Several characters gather at Nascita, a café owned by Sōichi Isurugi which he lives at with his daughter Misora.

Sōichi Isurugi is the enigmatic 48-year-old owner of Nascita and Misora's father, who is secretly a high-ranking member of Faust. Ten years prior to the series' events, he was the astronaut who discovered the Evol-Driver and the Pandora Box on Mars. This resulted in the alien entity Evolto possessing him and using him to commit numerous atrocities, including the Skywall Disaster. Sōichi's personality occasionally resurfaces to briefly fight Evolto before being suppressed. Sōichi was tricked into thinking that Evolto would leave his body upon retrieving the Evol-Driver, and ended up in a coma while Evolto possessed Ryuga and then Sento before creating his own body in Sōichi's image.

While possessed by Evolto, Sōichi utilizes the Fullbottle in conjunction with the to transform into . While transformed, he wields the dagger, which can combine with the Transteam Gun to form the .

Sōichi Isurugi is portrayed by .

Sawatari Farm is the Sawatari family's potato farm. It is located in Hokuto, the northern region of Japan isolated by the Skywall Disaster.

Kazumi Sawatari is a 29-year-old soldier from Hokuto and the current owner of his farm. In hopes of obtaining financial support, he volunteered for Hokuto's military and underwent human experimentation by Faust. He first infiltrates Touto during the events of the crossover film Kamen Rider Heisei Generations Final: Build & Ex-Aid with Legend Rider to reclaim the and Fullbottles for Hokuto. He is later deployed to lead the Hokuto army to invade Touto, and after Seito's invasion of Hokuto, he joins forces with those from Nascita in hopes of reclaiming his country.

Utilizing the Robot stand-up pouch in conjunction with the belt, Kazumi can transform into . While transformed, he wields the , which can switch between its raygun-like and its dagger-like . He can also use the Dragon Sclashjelly to produce a second Twin Breaker for dual wielding.

During his final confrontation with Evolto, Kazumi receives the cryokinetic from Sento, who warns him to only use said device as a weapon, as transforming with it would kill him due to a recent overdose of Nebula Gas damaging his body. However, he ignores the warning as he combines a copy of the Build Driver and the Grease Blizzard Knuckle while using the Fullbottle in conjunction with the latter to transform into his final form, , resulting in his death once the fight is over.

The new world version of Kazumi regains the memories of his alternate self when the Pandora Box is reconstructed and once again becomes Kamen Rider Grease once again. Unlike his original counterpart, he is able to become Grease Blizzard without the risk of death. After the events involving Downfall, Kazumi enters a relationship with Misora. During the events of the V-Cinema Build New World: Kamen Rider Grease, he obtains the device, which he can combine with the Build Driver while using the Fullbottle in conjunction with the former to transform into .

Kazumi Sawatari is portrayed by .

Hokuto Three Crows The are a trio of Hokuto soldiers who were originally workers of the Sawatari farm who enlisted to repay a debt to their former employer. Using Shinobu Katsuragi's Lost Fullbottles, they are each able to transform into an enhanced form of the Smash known as and later the stronger . After Sento and Ryuga are transported to the new world, the Hokuto Three Crows regain the memories of their alternate selves and their ability to transform while fighting alongside Grease against Downfall.

Akaba , real name , uses the Fullbottle to transform into the . He seems to possess a violent streak, as he attacks civilians without hesitation, but is loyal to Kazumi and follows his decisions even when he questions them. When Seito invades Touto to steal the Pandora Box, Akaba sacrifices himself to protect it from Gentoku, but ultimately fails to prevent its theft.

Akaba is portrayed by .

Aoba , real name , uses the Fullbottle to transform into the stag beetle-themed . Despite his looks and mannerisms, he is one of the kinder members of the Three Crows. He is killed in battle when Build goes on a rampage and uses the Hazard Finish against him.

Aoba is portrayed by .

Kiba