Nariaki Utsumi

"I swear my loyalty to you!"

- Nariaki Utsumi goes insane and swears loyalty to Evolt.

Nariaki Utsumi is a major antagonist in Kamen Rider Build. He is the assistant of Gentoku Himuro at the Touto Institute of Advanced Matter Physics and a member of the organization Faust. He later joins Namba Heavy Industries Ltd. after being used as a scapegoat by Himuro and becomes the assistant of Namba. Eventually though, Utsumi joins up with Evolt and becomes Kamen Rider Mad Rogue.

Reporting to Gentoku
Utsumi is first seen informing Gentoku about Sento's job application for the Touto Institute of Advanced Matter Physics, stating that Sento had passed the entrance test with a perfect score.

Utsumi is Sento's direct superior at the Institute, reporting directly to the Prime Minister of Touto, Taizan Himuro, while at the same time also secretly assisting Gentoku/Night Rogue within Faust. He was one of the first present witnessing Taizan's collapse, after the Pandora Box was stolen from the Institute. The Box was later hidden within a secret basement at one of Namba Heavy Industries' laboratories.

The Fall of Faust
After the Pandora Box was stolen by Blood Stalk, Night Rogue and Juzaburo Namba resolved to dissolve Faust by staging an operation to destroy it where Touto would supposedly take down Faust. In the operation Utsumi assumed the identity of Night Rogue and was exposed by Build as Faust's apparent leader, but Build remained suspicious.

Cornered by Build a bridge across a river, Utsumi revealed to Build that he had known for a while he was really Sento. He told Sento to "turn back" while he still had time but was shot by Himuro and knocked off the bridge before he could finish.

Working for Namba
However, Nariaki was later found by Blood Stalk and recruited into Namba Heavy Industries Ltd. He later revealed to Himuro that he had survived while he was waiting for Juzaburo Namba.

After Himuro was banished from Touto, he approached Utsumi and asked to be made a Kamen Rider, Utsumi made him beg for it, but then later declined his request anyway. Stalk later decided to make Gentoku into a Kamen Rider on his own, an idea that Utsumi did not like but still went along with.

Collaborating with Seito
Utsumi later updated the Kaiser System to create the Remocon and Engine Bro's to be used by Seito's military. For his efforts, Utsumi received a personal thanks from Seito's Prime Minister, Masakuni Mido.

He accompanied Blood Stalk to the meeting with Sento and Ryuga to exchange Taizan Himuro for the Evol Driver, though when Sento and Ryuga refused to follow through and fighting broke out, Utsumi fled the scene. He later ambushed Misora to steal the Evol Driver from her and gave it to Stalk, allowing him to become Kamen Rider Evol.

Later, as Kamen Rider Evol Dragon completed and used the Pandora Box to create Pandora Tower, Nariaki went out untransformed and demanded he stop opening Pandora's Box. Evol responding by shooting him away by with the Nebulasteam Gun. He later reappears back at Namba's office when Evol goes to meet with Namba.

Becoming Kamen Rider Mad Rogue
"Now, fight to your heart's content... Kamen Rider Mad Rogue!"

- Evol christens Utsumi's Kamen Rider form. After assuming his Black Hole form, Evolt goes to Namba in the form of a copy of Soichi Isurugi and declares to Namba that he will be the new leader of Seito, supplanting Namba Industries. He later meets with Utsumi and proposes that he come work for him, complimenting his skills as a scientist and revealing a second Evol Driver for him to use, though Nariaki is reluctant to take Evolt up on his offer.

Nariaki later meets with Gentoku Himuro and pleads with him to help them stop Evolt, saying he could destroy the enter world if not stopped. However, Gentoku is reluctant to trust his old assistant.

Nariaki eventually goes to confront Evolt alongside the Remocon and Engine Bro's and a squad of Hard Guardians. He produces a tablet for Namba to talk to Evolt with, and his boss proclaims his refusal to go along with Evolto's plans. Evolt then assumes his Black Hole form and teleports everyone to a barren battlefield for them to fight. Nariaki sends the Hard Guardians to attack first, but Evolt cuts them all down in a matter of seconds. The two Kaisers then go to attack, but fair little better against Evolt's immense power. The Kamen Riders then arrive and fight Evol alongside the forces of Namba Industries, though even with their combined power they are unable to hold their own against Evol.

After killing the two Washio brothers, Evol opens up a portal to Namba's office and goes through it to kill Namba. Utsumi could only watch in horror through his tablet as Evol disintegrated Namba's body. Returning through the portal, Evol restates his offer to Utsumi and tosses him Namba's old cane. Remembering his time as a Namba Child, Nariaki laughs and declares that, since Namba is dead now, nothing matters anymore and takes Evol up on his requests. Evol then gleefully tosses Utsumi the second Evol Driver, which Nariaki inserts the Bat and Engine fullbottles in to become a Kamen Rider, which Evol proclaims to be "Kamen Rider MadRogue".

Defecting to Evolt's side
With his newfound power, Utsumi fiercely attacks Kamen Rider Rogue and knocks him out of his transformation, refusing to listen to Gentoku's attempts to reason with him and proclaiming that with Evolt he finally has a chance to showcase his brilliance. As Ryuga rushes over to check up on Gentoku, Mad Rogue charges up an attack to finish them off with, but the attack is shielded by Kamen Rider Grease, who promptly retreats with the two injured Riders.

Back at the Pandora Tower, Utsumi is informed by Evol that now that Namba is dead, he will be assuming the identity of Masakuni Mido to propel the invasion forward, stating that reuniting Touto will make things easier for him in the long run. Utsumi then inquires to Evol what he intends to do about the Kamen Riders, to which Evol replies that he still intends to keep on using them to further his plans, instructing Utsumi to bring them to him alive. Admiring the Evol Driver, Utsumi expresses that he had previously believed their to only be one and asks Evol where the second one came from, but Evol instructs him to keep his questions to himself.

Visiting the ledge where he was shot off, Utsumi reflects on if he is wrong, but comes to the conclusion that he is not and chooses to reject the notion completely. Later, Utsumi leads an army of Hard Guardians as they assault a Touto city, only for Kamen Riders Cross-Z Dragon and Grease to show up and fight off the Guardians. Utsumi then transforms into Kamen Rider Mad Rogue and begins attacking the two, soundly overpowering them both. Takumi Katsuragi then arrives to help out, transforming into Kamen Rider Build, but Mad Rogue defeats him too and knocks him out of his transformation. However, spurred on by Ryuga and Gentoku, Takumi regains his memories as Sento, allowing him to use the Genius Fullbottle to transform into Kamen Rider Build Genius Form, shocking Mad Rogue.

Build Genius then overpowered and easily defeated Mad Rogue, but before he could finish Mad Rogue off, Kamen Rider Evol showed up and claimed the Hokuto Three Crows' Fullbottles from Kazumi before teleporting away with Mad Rogue. Back at Faust's lab, Mad Rogue revealed to Utsumi that he intended to wipe out humanity by turning them all into the final form of Smash, Lost Smash, and having them rampage until they've killed each other and destroyed the planet. He then confirms Utsumi's assumption that he left the Kamen Riders alive so he could turn them into Lost Smash.

Assisting Evolt
When Kazumi attempted to infiltrate Faust's lab to claim back the Hokuto Three Crows' Fullbottles, his disguise was seen through by Utsumi, who had a quartet of Faust Guardians capture him. Utsumi then subjected Kazumi to Nebulas Gas experimentation, making it so that if he ever lost a fight he would die. Kazumi manages to escape but is pursued by Faust Guardians. Finding himself face to face with Yoshiko Tajimi, Kazumi's Prime Minister uses the Owl Lost Bottle to transform into Owl Lost Smash to fight him. Build then shows up to assist Kazumi, but Mad Rogue also arrives and attacks Grease while Build engages the Owl Lost Smash. Spurred on by Build's determination to save the world, Grease fiercely attacks Mad Rogue and overpowers him, forcing him to fall back.

Kamen Rider Mad Rogue later participated in an attack on the Touto Government Office spearheaded by Black Hole Evol. He fought against Kamen Rider Cross-Z Magma and was fiercely beat back by him after he flew into a rage upon experiencing visions of Evol destroying Mars. After Evol annihilated the Touto Government Office, Mad Rogue retreated along with him.

Utsumi was later present when Evolt was giving a speech disguised as Mido to the people of all three territories about the reunification of Japan. After Evolt was done, he had the drone filming him turned off and told Utsumi to procure the Kamen Riders so they could use them to produce Lost Smash. Utsumi then told Evolt that having the Lost Smash attack people would bring about the downfall of the nation and questioned Evolt as to what his goals really were, but Evolt simply told him not to ask anymore questions if he valued his life.

Mad Rogue later pretended to fight off the Kamen Riders, who had been declared "terrorists" by the Seito media and slandered as attempting to oppose Seito's new order, in order to lure them out. Sento and Gentoku soon arrived to confront him, transforming into Build Genius Form and Kamen Rider Rogue respectively. Utsumi sent the Stretch Clone Smash he was with to attack Build while he fought Rogue. Mocking the Rider for being unable to live up to his father, Mad Rogue beat Rogue down though, inspired by Build's determination, Rogue refused to give up. After the Stretch Clone Smash was destroyed, Mad Rogue was defeated by the combined power of Rogue and Build and knocked out of his transformation. Utsumi later fled when Another Build arrived and ambushed Sento and Gentoku.

Noticing that Gentoku and Kazumi were sneaking around Faust's base, Utsumi captured them and had them dosed with a high concentration of Nebula Gas so they could be turned into Lost Smash. However, the two manage to escape before the procedure was finished, prompting Utsumi to send Faust Guardians after them, as well as transforming into Kamen Rider Mad Rogue to fight them. Kamen Rider Build then arrives in his TankTank form and fights off Mad Rogue and his Guardians while Cross-Z Magma flies away with Gentoku and Kazumi. After Mad Rogue gets knocked to the ground by Build, Shinobu Katsuragi shows up as Build NinninComic to fight Build instead and causes Sento's Build Driver to teleport him away. He then tells Utsumi that Build escaped.

After Kamen Rider Evol was defeated by the Kamen Riders, he sent some of his DNA to possess Utsumi in addition to hiding it in the Evol-Driver, anticipating that Shinobu Katsuragi would betray him. After Shinobu brought the Riders to the Pandora Tower and revealed he was working against Evolt the entire time, Evolt showed up and revealed he was possessing Utsumi before fatally wounding Shinobu and transforming into Kamen Rider Mad Rogue to fight the Riders. Evolt quickly found Utsumi's power to be limiting though, and discarding him so he could retake his own Evol-Driver and use it to become Kamen Rider Evol.

Later, after Evol lost two of the Lost Fullbottles to the Riders, Utsumi walked up to him and accused him of only conquering Japan to buy himself time to complete the Black Pandora Panel. Evol responded by grabbing Utsumi by the neck and telling him to "shut up", threatening to kill him if he revealed his true intentions to the Riders.

Following his announcement of a final battle against the Kamen Riders over live television, Evolt had Utsumi run simulations of a Strong Smash Hazard and a Flying Smash Hazard so he could practice against them. After destroying both with single strikes, Evolt remarked to Utsumi that he would make a good replacement for Shinobu.

On the day of the final battle, Utsumi stayed behind at the top of the Pandora Tower to watch the Kamen Riders as they made their way through the tower.

Personality
Utsumi typically shows a stoic demeanor, silently going along with whatever Gentoku or Namba tells him to do. But, as the series goes on, Utsumi is shown to be more maniacal than he first appears. When Gentoku pleads with him to make him a Kamen Rider after being exiled, Utsumi reveled in the fact that he now held power over his former boss and enjoyed making him kneel as revenge for Gentoku using him as a scapegoat.

As a Namba Child, Utsumi was raised and indoctrinated to be completely loyal to Namba. However, this does not meet he doesn't have any compunctions about his boss's actions, as he was shown to be unnerved when Stalk killed Masakuni Mido. He also was wary of Evolto and his motives for working with Namba, not fully trusting him.

After Namba is killed by Evolto, Utsumi goes completely insane and pledges himself to serve Evolt. According to Gentoku, he also had a part in Utsumi's descent into madness, as the event where he was shot down under the bridge by the former served as the catalyst to his currently maniacal behavior.