This is a partial character sheet for Kamen Rider Build. Visit here for the main character index. Subjective trope and audience reactions should go on the YMMV page.
Because of the nature of the characters on this page, it is impossible to discuss them in detail without spoiling major elements of the story. Unmarked spoilers below.
- Faction Calculus: The Powerhouse faction, with only two named combatants in the war but those two being able to build up their strength at a far faster rate than anyone else.
- Government Conspiracy: Every single named character at the Touto Institute of Advanced Matter Physics either is or has at one point been a member of Faust. The criminal organization is in essence a Touto military project with a layer of plausible deniability.
- Technical Pacifist: While spouting the ideas of pacifism, Touto officials have no qualms about using deadly force against our heroes.
- Tokyo Is the Center of the Universe: Touto encompasses the former capital of Japan, Tokyo. It's also the main setting of the show.
Portrayed by Meikyo Yamada
The Prime Minister of Touto and Gentoku's father. Unlike the leaders of Seito and Hokuto, he ultimately wishes for Japan to be whole again in a way it was before the Sky Wall Incident, not the war of conquest they want.
- Big Good: He's the only regional leader who's actively trying to fix things. After Evolt is destroyed and for good and the world is remade as if he never influenced Earth in the first place, Taizan is now the Prime Minister of Japan as a whole.
- Good Is Impotent: He's the only Prime Minister who isn't a cartoonish villain driven mad by desire for the Pandora Box, but Taizan isn't fully aware of Faust's activities within his borders, nor how thoroughly they've infiltrated the Touto Institute for Advanced Matter Physics. Several incidents conspire to put him out of commission at nearly any time that he'd actually be useful, and although he eventually starts to avert this, it's far too late to stop the war.
- Good Parents: He loves his son and wants him to know it, but considering how Gentoku is a power hungry maniac, his attempts usually fall flat. Granted, he did disown Gentoku after getting to known about everything, but that was completely justified shock. He tries to talk it out the next time they meet even thought he is held at a gunpoint.
- Heroic Sacrifice: How he dies. Defending Gentoku from Utsumi worked out, but protecting him from Evol was fatal.
- I Have No Son!: He tells this to Gentoku in #20, exiling him from Touto, and recants it with his dying breath.
- It's All My Fault: He blames himself for Gentoku's descent into evil. He couldn't make it to the Mars Return Ceremony and so he sent Gentoku in his place, causing him to be affected by the Sky Wall incident.
- Locked Out of the Loop: He doesn't know anything about Gentoku's involvement with Faust until #15.
- Killed Off for Real: He dies taking the hit meant for Gentoku.
- Papa Wolf: He may be a frail old man, but given a proper chance and desperation, he will protect Gentoku. His escape from Namba's prison is nearly foiled by Utsumi, who shocks Gentoku with the killswitch for his treachery. This leads to Taizan body checking Utsumi and knocking the remote from his hand.
- Put on a Bus: Taizen suffers repeated incidents that take him out of action, allowing Gentoku to push ahead with his plans while his father's unable to stop him.
- Reasonable Authority Figure: Wishes for Japan's reunification and is against Gentoku's plan to enlarge Touto's military. Justified, as he's the only regional leader that wasn't present at the Skywall Tragedy, and thus not affected by Pandora's Box's powers. By episode 15, the heroes seem to have earned his trust once he catches Gentoku red-handed, but unfortunately he gets put in a coma by Blood Stalk immediately after. Five episodes later, though, Misora's new Healing Hands have him back in action, and he promptly disowns Gentoku and kicks him out of the country.
- Taking the Bullet: He runs to take the hit meant for his son and dies.
- Token Good Teammate: For all the regional leaders, due to having not been exposed to Pandora's Box during the Skywall Tragedy.
Portrayed by Kensei Mikami
The head of the Touto Institute of Advanced Matter Physics as well as an aide and son to the Prime Minister of Touto, Gentoku oversees the research on the Pandora's Box. Was present at the Skywall Tragedy 10 years ago, and thus was also affected by its energies, increasing his aggression.
He is the true identity of Night Rogue, one of the creators of Faust. After being expelled from Touto, he becomes a servant of Seito, using the Crocodile Crack Bottle with the Sclash Driver to transform into Kamen Rider Rogue.
See his page in Kamen Rider Build Protagonists
Portrayed by Yuki Ochi (live) Yuji Nakata (suit)
The secretary to Gentoku Himuro and another aide to the Prime Minister of Touto, Utsumi is rarely seen without his boss. After seemingly being killed by Gentoku as a substitute Night Rogue, he's saved by Stalk and recruited to work for Namba Heavy Industries. In this role, Utsumi develops Hokuto and Seito's Sclash Drivers, recreates the Kaiser System to create the Bros, and takes the original Transteam Gun for himself to become the second Night Rogue. He later uses the Evol Driver together with the Bat and Engine Fullbottles to become Kamen Rider Mad Rogue after being driven insane by Evolt.
- Ace Custom: Zig-Zagged. Utsumi's Evol Driver, while visually identical, had been modified and tuned for human use. While it gives him greater initial power compared to users of the Build System, it has a lower limit than Evolt's original.
- Alien Geometries: While Build and Cross-Z's transformations include straight, even "pipes" that distribute the Nebula Gas, contained in mechanical devices that carefully weave them into the shape of the armor, Mad Rogue's is a chaotic, random tangle of pipes that converges down on Utsumi's body to complete the transformation.
- Because You Were Nice to Me: It's implied in the YouTube specials that he has a soft spot for Sento because the latter sees him as a fellow scientist, rather than just the weapons factory that Namba and Night Rogue treat him as. However, this does nothing to stop his loyalties to Namba Industries or prevent him from using the data he gathered on Kamen Rider Build to perfect the Hell Bros.
- Big Damn Heroes: Shows up when Gentoku is about to fight the Mimics Evolt generated of the Hell Bros, for what Gentoku thinks is making the fight 3-on-1. Then he promptly turns on the Hell Bro Mimics, at least entering an Enemy Mine with Gentoku.
- Body Backup Drive: The real reason Evolt gave him a copy of the Evol Driver was to make it so that he could use Utsumi as a host body in the event of his "death".
- Body Double: Is turned into one for Night Rogue by the real deal.
- Brainwashed and Crazy: As a Namba Child he's been conditioned since childhood to be absolutely loyal to Namba himself. This works in Evolt's favor after he kills Namba, as Utsumi's unstable mental state causes him to snap and swear loyalty to Evolt instead.
- Curb-Stomp Battle: After becoming Kamen Rider Mad Rogue for the first time he fairly easily beats down Kamen Rider Rogue. Later on, he winds up on the receiving end of this from Build Genius Form.
- Crazy-Prepared: He designed Hell Bros with a deliberate weakness to the SameBike Best Match. He had no reason to ever think that Hell Bros would be used against him or Namba, but it pays off much later when he and Gentoku team up against a Mimic created by Evolt.
- Creative Sterility: While he's been shown to be great at reverse-engineering (like what he did with Katsuragi's Rider System) or improving upon something (like what he did with Kaisei Mogami's Kaiser System), in all cases, he's always shown relying upon the inventions of others, unlike Sento, who created most of his own inventions even if he doesn't remember making about half of them. Even the Sclash Drivers he made for Hokuto and Seito weren't his, being designed by Katsuragi. Finally, when giving the Kaiser Brothers their Deadly Upgrade, the process is almost exactly like the one used for the Hokuto Trio, the only difference being that Utsumi was able to do it without the Hazard Trigger.
- Crouching Moron, Hidden Badass: After becoming Mad Rogue he spends most of his time either getting his ass kicked to show off new forms and power-ups or generally just a slightly more powerful enemy to fill out fights. In the third-to-last episode he reveals that he was faking loyalty to gather data on Evolt's Phases to become even more powerful and avenge Namba. He actually keeps Evolt up against the ropes for a while before he gets too cocky and activates Phase 4, blowing out his own cybernetics.
- Decoy Getaway: Utsumi briefly becomes Night Rogue to throw suspicion off of Gentoku, and gets shot for his trouble. He makes use of the trick himself as the second Night Rogue by using his Smash holograms to distract SmartphoneWolf Hazard by eating its finishers while he plucks the Hazard Trigger right off of Sento's belt.
- Despair Event Horizon: After Evolt kills Namba, Utsumi leaps over the edge, cracks a goofy grin and gleefully swears loyalty to Evolt, becoming Mad Rogue.
- Dirty Coward: Will gladly double cross others and turn his back on ideals to save his own skin. And when he's on the losing end against the heroes, he'll panic and hide behind anything to save himself.
- The Dragon: After becoming Mad Rogue, he is effectively this to Evolt, acting as his enforcer and doing his dirty work whenever he doesn't feel like getting involved.
- Dragon Their Feet: While acting as Namba's assistant for a portion of the series, after Namba gets offed by Evolt, Utsumi goes on to serve him.
- Even Evil Has Standards: Played with. Juzaburo Namba's actions and escalating ambitions after the Seito representative duel leave Utsumi visibly unnerved, but it's not enough to overcome the brainwashing he's been indoctrinated with since childhood, and so he continues to go along with Namba's plans even knowing that what he's doing is wrong. He even takes a sort of twisted satisfaction in being put in a position where he can repeatedly take out his frustrations on Gentoku.
- Evil All Along: Turns out to have been one of Namba's top men from the beginning, even before the Skywall incident.
- Evil Counterpart: He becomes one to Kamen Rider Rogue (who has reformed by that point) after obtaining his Mad Rogue form.
- Evil Genius: Of Namba Heavy Industries, being their most capable engineer. He creates Sclash Drivers for both Hokuto and Seito and reverse engineers the Kaiser System.
- Evil Makeover: Eviler makeover, that is. Utsumi wears Seito's colors and changes his hairstyle after joining Evol.
- Evil Is Petty: When Gentoku threatens him at gunpoint to make him a Rider, he forces his old employer to kneel down and beg for him to do so. And then he doesn't.
- Fatal Flaw: His reliance on having superior weaponry and data. As Mad Rogue, he mainly relies on his Evol Driver boosting his capabilities. However, it's never enough to truly close the gap between him and the heroes since not only do they have higher Hazard Levels and/or equivalent technology, they also have much more combat experience than him. In the end, his over reliance on the Evol Driver costs him dearly when he pushes his Evol Driver to Phase 4 in order to deal the finishing blow to Evolt only for the power to be too much for his cyborg body, interrupting him at the worst possible moment.
- Flanderization: Utsumi's appearances in Build new world are mostly internet memes made Ascended Canon.
- Four Eyes, Zero Soul: Always seen wearing a pair of glasses, though he doesn't fully embrace the "Zero Soul" part until he goes crazy and joins Evolt. Somewhat subverted when it's revealed he was just biding his time until he could get revenge for Namba's death.
- Forced to Watch: All he can do as Evolt kills Namba is helplessly watch over his tablet's live feed. This directly leads to his Sanity Slippage and becoming Evolt's Dragon.
- Go Mad from the Revelation: The death of his "father figure" Juzaburo Namba in Episode 38 seems to drive him over the edge. Evolt shows up and throws him Namba's walking stick while asking him if he wants "Do you want revenge for Namba, or to swear your loyalty to me?" Utsumi flashes back to memories of his time under Namba...and then bursts out laughing, breaks the cane over his knee, and signs up with Evolt with a grin on his face. Appropriately enough, his new form is named Kamen Rider Mad Rogue. It's later subverted when it's revealed that this was all an act just so he'd have a chance to avenge Namba.
- Good All Along: Er, Less Evil All Along, but it's revealed in episode 47 that he was only pretending to be on Evolt's side, if only for the chance to gather data to use against Evolt to avenge Namba. He still holds a grudge against Gentoku for what happened earlier, but he's more than willing to put aside that grudge if it means the chance to take down Evolt.
- Green-Eyed Monster: He's smart, but he's resentful of the fact that he's relied on not for his own unique inventions, but for his ability to copy the creations of superior geniuses such as Sento, Katsuragi, and Kaisei Mogami.
- HeelFace Door-Slam: Subverted. He tries to convince Sento that there was another path to take and to not end up like he has, but is shot by Gentoku before he could say anything else. He survived, but rather than turn face, returns to working with Namba Heavy Industries. He looks like he might turn again as Namba's ambitions escalate into madness, but his indoctrination is too strong for him to do more than look a bit worried before going along with whatever his master desires.
- Heroic Sacrifice: Sacrifices himself to protect Gentoku and Sawa against a blow from Evolt. However, he's only protecting the Lost Bottle, since he still hasn't entirely forgiven Gentoku.
- Hidden Depths: Episode 12 implies that his involvement with Faust was not entirely willing. Given later reveals about him he might have actually been talking about being a Namba Child.
- I Fight for the Strongest Side: Utsumi has no real will on his own and usually falls in line with whoever is directly in charge at the time such as Gentoku, Namba, and ultimately Evolt.
- I Just Want to Be Normal: Breaks down near the end of #12 when he confesses to Sento that all he ever wanted was to live a normal life.
- In-Series Nickname: Sento jokingly calls Utsumi "Cyborg Utsumi" in the opening recap for episodes 19 and 39. Turns out that was Foreshadowing to the fact that Utsumi survived being shot #12 by being made into a Cyborg.
- Jacob Marley Warning: He warns Sento not to become like him in #12. He doesn't get to say what it's supposed to mean before Gentoku shoots him.
- Laughing Mad: His descent into Evol's mad enforcer is accompanied by a marked increase in insane laughter.
- Meaningful Name: His Kamen Rider form is called Kamen Rider Mad Rogue. He's not the sanest guy around after his Sanity Slippage.
- Not So Different: Sees himself this way to Sento, and attempted to convince the latter not to end up like him before his death.
- Not So Stoic: He smiles for a moment and cracks a joke at Sento in #12. His breakdown at the end of the same episode counts too.
- One Steve Limit: Averted. His Mad Rogue Bat FullBottle is visually and functionally distinct from the one he and Gentoku used to transform into Night Rogue. When put into a Driver, the sound byte played is even in Japanese instead of the typical English the belts spout off when loaded with bottles.
- Only Sane Man: With the obvious exclusion of Sawa who is the only one who got out the sinking ship that is Namba Heavy Industries. Utsumi seems to be the only villain to openly question Evolt's methods. It's then blown straight to hell once Utsumi joins Evolt and becomes the oh so fittingly named Mad Rogue.
- Sadist: He very clearly enjoys crushing Gentoku's hand under his shoe as he's begging to be made a Rider, and then chains his old boss up while denying him the privilege. Considering what Gentoku did to him, can you blame him?
- Sanity Slippage: Utsumi brings a whole new meaning to Jumping Off the Slippery Slope with the series of events that led to him becoming Mad Rogue. He goes from calm and collected to Laughing Mad in under a minute.
- In Build new world there is nothing left of Utsumi. And recovering his memories has reduced him from a loyal worker to a living memeof a man.
- Satellite Character: The majority of his character revolves around his role as Gentoku's secretary, until his apparent death in episode 12 and his return in episode 18, at which point he switches to being Juzaburo Namba's satellite instead. Come episode 38, he's now Evolt's.
- Spanner in the Works: During The Movie, he ends up breaking Gentoku and Kazumi out of Kengo's captivity, allowing them to help Sento. Unsurprisingly, he makes them beg for his help first.
- The Stoic: Utsumi rarely allows himself much in the way of expressions, making his breakdown in episode 12 all the more shocking.
- Suicidal Overconfidence: He charges Evolt with nothing more than a Transteam Gun untransformed and demands he stop opening Pandora's Box. He gets casually sent flying for his trouble.
- Took a Level in Badass: He actually gets to fight as Night Rogue in the YouTube specials, showing himself to be a great deal more cunning and analytical than Gentoku ever was. He later gets an even bigger upgrade after receiving an Evol Driver (which explicitly needs a Hazard Level of 5) from Evolt to become Kamen Rider Mad Rogue.
- Would Hit a Girl: Attacks Misora to snatch the Evol Driver from her.
Tropes exclusive to him as Kamen Rider Mad Rogue:
- Bat Out of Hell: His Animal Motif.
- Dual Wielding: Prefers to use both the Nebula Steam Gun and Steam Blade in battle at the same time.
- Explosive Overclocking: He was able to imitate Evol's various Phases, even up to Phase 4. However, this was too much for his cyborg body, and it results in his death.
- Finishing Move: The Evoltech Attack seems to be whatever he desires it to be. To wit, he's performed a flying dropkick, a Kick Them While They Are Down variant, a shot with a rifle, and even a version where he flew around like a bomber.
- Flight: He can fly using the Mad Night Flyers that sprout from his back.
- Guns Akimbo: Can use both the Transteam Gun and Nebula Steam Gun at the same time.
- Glass Cannon: As Kamen Rider Mad Rogue, his Evol Driver compensates for his low Hazard Level, but it only manages to put him on par with the protagonists with even the bottles bequeathed to him being regular ones from Pandora's Box.
- Power Copying: Like Evol, Mad Rogue can use the Rider Evolbottle in conjunction with a Fullbottle in order to get the corresponding Build weapon.
- Strong, but Unskilled: Zig-zagged. Utsumi is still a dangerous fighter, but he's still reliant on his combat data and the power boost from the Evol Driver, but the other Riders are more skilled than he and have fought longer. The moment someone gets a new trick or equals him in power, he falls flat on his ass.
Portrayed by Yukiaki Kiyama
The researcher originally in charge of studying the Pandora's Box, a brilliant man known as "The Devil's Scientist" by his colleagues. Ryuuga was arrested for his murder prior to the series, and discovering the mystery behind Katsuragi's murder is his main motivation.
He was a founding member of Faust, the original planner of Project Build, and most importantly, the true identity of Sento Kiryu.
- Anti-Hero: Once his full motivations are revealed, it's clear that Katsuragi was seeking a good end, but for vengeance rather than justice. Unlike his current personality as Sento, Katsuragi is willing to get his hands dirty in order to defeat the likes of Kaisei Mogami and Evolt, even as far as to steal Ex-Aid's powers or try to kill the yet-to-be awakened Ryuga just to achieve his goals, but that same willingness to do anything to kill Evolt was what let the snake take advantage of Katsuragi's brilliance to help get his plans rolling.
- Back from the Dead: Either as a side-effect of his possession or as a deliberate act of spite, Evolt's theft of Sento's body causes Takumi's memories and personality to return and Sento's to vanish. It only lasts for a few episodes before Takumi willingly "dies" a second time.
- Catchphrase: Episode 39 shows that Sento's catchphrase "Isn't it great? Isn't it amazing? Aren't I a genius?" started with Katsuragi, but he delivers it in a much more arrogant manner.
- Cold Ham: He loves science just as much Sento, but since he is stoic and reserved compared to his much more excitable other self, he naturally expresses it in a way as cold as to give someone a frostbite.
- In ROGUE special, he bolts towards the research equipment at Faust's base in the same way Sento typically reacts to stuff that interests him, but his expression doesn't change nor does he say anything.
- He has the same victorious routine after inventing something as Sento has, but performs it without any childishness or excitement.
- Comfort Food: Tamagoyaki note prepared by his mother. He apparently liked it so sweet that it's unbearable for anyone else to eat.
- Creative Sterility: Of a sort. He's certainly not starved for ideas, but he's so stuck in his ways and distrusting of others that he can't properly use Build Genius and ultimately can't utilize the Build System itself because his emotional mindset is too dulled to bring out its full power. His conversation with Sento in a representation of his mind shows formulas and gears stuck statically in place that only start moving again once he puts Sento back in the driver's seat. The real fault in his creativity, however, is one he shares with Sento: they both respond to problems by building weapons instead of thinking about how to use the Rider System and its components constructively, which blinded them both to the real functionality of the Hazard Trigger.
- Dissonant Serenity: His eager and professional demeanor never falters during his device showcase videos, even when he's talking about how some of them have lethal, mind-destroying side effects. This partially carries into Sento's habit of using this state as a defense mechanism in traumatic situations.
- Everyone Has Standards:
- Even for Katsuragi, the Hazard Trigger was too much, as was the discovery of Stalk and Ryuga's shared true identity. The real reason he brought both to his house was to try and kill them.
- Near the end of the first part of the ROGUE miniseries, he's noticeably uncomfortable when Gentoku repurposes his underground research facility into the starting point of Faust.
- Heisei Generations FINAL reveals that he helped build Kaisei Mogami's Enigma device, and even used it to travel to the main Kamen Rider universe and steal Emu Hojo's powers, but the moment he found out about Mogami's plan to forcibly merge the two worlds (which would destroy them both) he tried to pull the plug.
- Foil: It's clear that he and Sento are two completely different people/personalities when you break it down.
- Relationships: Sento, despite his smug behavior, has complete trust in his allies, cares for Misora as a clear friend, and despite bickering with Ryuga several times, they are clearly best friends. On the other side, Takumi trusts nobody, sees Misora as nothing but a means to create FullBottles, and was actively trying to kill Ryuga due to the latter containing Evolt's DNA.
- Use of the Build Station. Even though Takumi built the Build Driver and powers, he was unable to handle the more powerful forms Sento created, and had to adjust the RabbitRabbit form for his own goals. Sento had at least a year to train with the Build system, went through multiple different Best Matches with different abilities, and was easily able to adapt to any situation.
- And finally, their beliefs. Sento wanted to use the Build system as a way to protect people, and believed that science should be used to help instead of destroying. Takumi wanted to use the Build system to defeat Evolt, which turns into a plot point when he could not use the Genius FullBottle as his desire was not compatible with Sento's sense of justice.
- Good Is Not Nice: When his memories return late in the series, his primary goal is to defeat Evolt to save humanity. However, he dismisses anyone of "lesser" intelligence, treating Sento's friends and allies as little more than gibbering chimps and refusing to hear any debate about sparing Banjou's life because they can't offer it in a logical manner.
- HeelFace Door-Slam: Played with. He attempted to quit Faust and kill both Blood Stalk and Banjou upon discovering that they were the mind and body of Evolt, only for Stalk to have outplayed him by swapping Katsuragi's Build Driver for the nonfunctional prototype. Stalk then wiped Katsuragi's memory and changed his appearance, turning him into Sento. He still ended up becoming a good guy, but in a way that, initially, served Stalk's plans rather than ruining them.
- He's Back: In the absolute worst way. Raising his Hazard Level into the 6.0 range and subsequently being possessed by Evolt restored his memories at the cost of Sento's personality disappearing.
- How Do I Shot Web?: To his frustration, he's barely able to use Sento's upgrades due to his lack of experience despite having been the major architect of the entire Rider System.
- Like Father, Like Son: Turns out his father was also involved in human experimentation, with Ryuga as a test subject.
- Last Chance to Quit: The usually rabid and reckless Gentoku gave him ample opportunities to scrap Faust before they went past the point of no return, but Katsuragi pressed on anyway, believing their experiments, as monstrous as they would inevitably become, would benefit human civilization in the long run.
- Mad Scientist: He was the first to conceive of Nebula Gas technology, and at least for some period he was entirely comfortable with human experimentation. It still shows up in his new personality, Sento Kiryu, but to a lesser degree.
- My God, What Have I Done?: After his return, he dissolves his relationship with Gentoku once he learns that in spite of all they had sacrificed to create the Rider System, the nightmare scenarios they tried to prevent with its usage (Evolt's return to power and a Japanese civil war respectively) still came to pass.
- Not So Different: The more the audience learns about Katsuragi, the more it's revealed that he's not far off in personality from his new identity as Sento.
- Not What It Looks Like: Heisei Generations FINAL reveals that he was the Build who appeared in Ex-Aid: True Ending and stole Emu's powers, seemingly being his usual "Devil's Scientist" self. Later in the movie, Sento finds Katsuragi's notes and discovers that he was trying to save Ex-Aid's powers, believing that Hyper Muteki was the only thing capable of stopping Mogami's plans but realizing that his powers would be affected by the two worlds merging.
- Posthumous Character: Initially alleged to be dead before the show even begins, before it's revealed that he was Sento all along.
- Power Incontinence: During his return and loss of his memories as Sento, Katsuragi realizes that he can't keep up with the current Build upgrades.
- Then Let Me Be Evil: It's shown in a flashback that Katsuragi accepted that his desire to advance the world through Faust and Project Build would lead to suffering, but such was his fury and resolve that he declared he would even sell his soul to the devil if it was for the sake of research.
- Reality Ensues: When he fights as Build in Episode 38, he can't handle the various upgrades, weapons and tweaks that Sento made in his "absence" as the furthest he got was GorillaMond. He also didn't realize that the Pandora's Box panel doesn't work as a power-up because it's meant to simply increase the user's Hazard Level fast enough for them to perform a Suicide Attack.
- The Red Baron: "Devil's Scientist".
- The Reveal: While the audience had plenty of reason to guess it beforehand, #15 finally confirms that Katsuragi lives on as Sento.
- Split-Personality Merge: Averted as the solution to his and Sento's situation: after Sento becomes their dominant personality again, he gets Katsuragi's childhood memories, but not the ones from his adulthood. Instead, Katsuragi's personality containing those memories sticks around as a Spirit Advisor.
- The Starscream: He worked with Faust while intending to kill Evolt from the very beginning to avenge his father.
- The Stoic: In life, he was phenomenally bad at expressing his emotions. He even kept an even, level face while eating his favorite food. That being said, it was just expressing those emotions he was bad at. He still dearly loved his mother, despite never telling her such, and left her a coded message in his final message thanking her for giving him life.
- Straw Vulcan: During his return in Episodes 37-39, he dispassionately dismisses the rest of the cast's silly antics and belief in friendship as nonsense straight out of a shonen manga while planning out a way to destroy Evolt...and Banjou, whom he believes will become another Evolt if left unchecked. However, despite having created the Genius Bottle himself, he's unable to use it because he doesn't have the right emotional mindset; after he regains all his memories and becomes the emotional Sento Kiryu again, the Genius Bottle works perfectly, creating Build Genius Form.
- Walking Spoiler: Just like Himuro, it's impossible to discuss his role in this series without spoiling huge plot twists. After all, he is our protagonist.
- Well-Intentioned Extremist: Katsuragi is implied to have had sympathetic motivations for creating Faust, but the monster he created grew far beyond his control. When he learned who Evolt was, he tried to kill him and his other host Banjou before he could set into motion his plan to destroy the world.
- Who's Laughing Now?: Furious that the nation drove his father to suicide by blaming him for the Sky Wall Disaster, he decided to reclaim his family's honor by creating weapons derived from the anomaly's Nebula Gas deposits.
- Would Hurt a Child: He exploited a preteen Misora's abilities dozens of times to create his FullBottes. His incredibly backhanded apology for it when he returns stuns the usually fierce girl with its sheer glib audacity, much to Ryuga's ire.
Shinobu Katsuragi / Kamen Rider Build
Portrayed by Joji Kokubo
In truth, before his supposed death, Shinobu was blackmailed by the newly-arrived Evolt into fixing the broken Evol Driver, and his research notes on Evolt, the Evol Driver, and the Pandora Box prove crucial to the creation of the Genius FullBottle. Supposed death, because evidence soon comes to light that like his son, rumors of Shinobu's death have been greatly exaggerated.
- Always Someone Better: He's this to Takumi and Sento, being able to not only repair the Evol Driver, but create a replacement EvolBottle for the one that broke on Mars, and even constructed a whole new Evol Driver almost as good as the original, though tuned down for a human. While the Genius FullBottle is made using Takumi's skills and Sento's purifier machine, it's built on a function of the Pandora Box that Shinobu discovered. He also created the Lost Bottles, although even Takumi can't begin to fathom why his father would have done something so vile. It's eventually revealed that they exist for a project similar to what Takumi had previously explored with Kaisei Mogami: to eliminate Evolt and undo his actions by creating a Merged Reality, using Evolt himself as the fuel source.
- Even as Kamen Rider Build, Shinobu's better than his son, able to curbstomp Cross-Z Magma with nothing but vanilla Best Matches when Banjou's Hazard Level has climbed to a degree far beyond what a human is even theoretically capable of.
- Ambiguously Evil: Shinobu's actions at first appear unquestionably evil, but soon contain enough shades of stealth mentoring Sento that it's clear there's more to Shinobu than meets the eye. He disables Build by striking the Hazard Trigger, then uses NinninComic's Smoke Out ability to teleport Sento to safety, all while delivering a speech that can be read as either Evil Gloating or a legitimate warning. Sento later realizes that he can use the same trick on the Evol Trigger and starts wondering if his father showed him the trick on purpose so he could use it against Evolt.
- Big Good: Shinobu eventually turns out to be a secondary one after it's revealed that he's Good All Along. While Vernage gave the heroes a fighting chance, it's Shinobu's actions that ultimately paved the way for Evolt's defeat. After making contact with Evolt, he elected to feign allegiance with him in order to study both him and the Pandora Box. Not only did this allow him to create the Rider System, but it also allowed him to learn of the Black and White Pandora Box Panels, allowing him to trick Evolt into sparing Earth for the time being with the promise of more power, and create a means by which Evolt could be destroyed and everything he did would be undone. While he never lived long enough to see the plan to through the end, he was able to entrust the plan to Sento, who was ultimately able to finish the job.
- Evil Counterpart: While not truly evil, Shinobu plays the role while being active on Evolt's side as another Kamen Rider Build.
- Famous Last Words: "Takumi... Have you gotten taller again?"
- Good All Along: As it turns out, he's been plotting against Evolt the entire time just waiting for his chance. Once Evolt's defeated by the other Riders he seals the Evol Driver in a canister to keep him from regenerating and reveals his plan to use the black Pandora Panel to do something to protect humanity. Unfortunately, Evolt had a backup plan and Shinobu is Killed Off for Real in retribution for his betrayal.
- Go Out with a Smile: He dies having passed on his plan to Sento, happy as he fondly remembers seeing his son before the Skywall Disaster and remarking on how he's grown taller.
- I Did What I Had to Do: He worked with Evolt even though his plans would kill a whole lot of innocent people because if Evolt won, it would mean that everyone would die. Thus, he stuck close to Evolt in order to plot against him right under his nose.
- Meaningful Name: Shinobu's name is written with a kanji which means "concealed" or "hidden", referring to the fact that he spent most of the series Faking the Dead. It's also half of the word shinobinote , and NinninComic is Shinobu's most-used form of Build.
- Not Brainwashed: Even though he was closest to the Pandora Box when it was first activated, he wasn't mentally affected like the attending leaders. It's implied that it's because the sheer devastation brought by the Sky Wall shocked him so much.
- Out-Gambitted: He planned to trick Evolt into losing on purpose in order to complete the last Lost Bottles and then trap his Evol Driver inside a canister in order to prevent him from regenerating, and then use the Black Pandora Panel's warp travel abilities to hopefully find a more permanent solution. Unfortunately, Evolt was already suspicous of Shinobu was smart enough to splinter off a part of himself to posses Utsumi, allowing him to confirm his suspicions, retrieve his Driver and claim the Lost Bottles and Black Pandora Panel.
- Becomes an Inverted Trope since he manages to get his son to complete his plans regardless of dying.
- Posthumous Character: Like his son, Shinobu is believed dead long before the show even begins. The truth is more complicated.
- So Proud of You: His last words are an echo of something he said to Takumi before the series began. In the context of him telling Sento however, it's expressing pride in how far his beloved son has come.
- Super Prototype: Shinobu uses the first prototype of the Build Driver to become Build. Combined with his high Hazard Level, Shinobu can hit hard, take more damage, and use stronger attacks that Sento never used before. As the creator of the Build Driver himself, Shinobu maximizes the power and effectiveness of his Build's forms, even with Build's standard Best Matches.
- Thanatos Gambit: Uses his last moments in order to tell Sento about the White Pandora Panel, effectively being the one responsible for what happens at the end.
Eita Kawai and Shingo Kuwata
Portrayed by Tsukasa Honjo and Ryo Yoshida
Two of Sento's coworkers at the Touto Institute of Advanced Matter Physics. Like almost everyone else there, both are secretly Faust agents.
- Cyanide Pill: In addition to the unpurified Full Bottle he uses to become the Flying Smash Hazard, Shingo carries an extra Full Bottle of raw Nebula Gas that acts as one of these, disintegrating him when used.
- Dramatic Irony: Shingo's last words are a declaration that Katsuragi is alive operating under the belief he shared with Eita that Stalk was a disguised Katsuragi. He's not only wrong, but he's saying this to Sento, who is Katsuragi.
- "Freaky Friday" Flip: Eita is given Shingo's face by Blood Stalk after accusing Stalk of pulling this trick to hide his alleged identity as Takumi Katsuragi. Which is what happened to Katsuragi, but he didn't become Stalk.
- Loony Fan: Their Undying Loyalty to Katsuragi borders on fanaticism. Shingo commits suicide by Nebula Gas overdose to avoid capture while declaring his belief and Eita is adamant that Katsuragi could never be killed like he seemingly was.
- Small Role, Big Impact: They last two episodes before being Killed Off for Real, but their fanatical devotion to Katsuragi spurs Sento into discovering the truth behind the Devil's Scientist.
- Those Two Guys: Episode 9 presents them as this, even getting their own namecards as if they're going to be recurring minor characters. It immediately pulls a bait-and-switch when both are revealed to be Faust agents and die by the end of the next episode.
- Undying Loyalty: Both are intensely loyal to Katsuragi as a genius scientist.
- A.I. Is a Crapshoot: Their focus is obscenely single-minded, and they aren't especially intelligent, such that they can all decide to only target Build even though there's an active and dangerous Smash right in front of them. Making matters worse is that Stalk, and presumably anyone else working with Namba Heavy Industries, can take control of any Guardian at any time. Subverted as the guy they're taking orders from was the guy sending out Smash to begin with and that's exactly what Namba designed them to do.
- Bayonet Ya: Guardian's rifles are adorned with a blade at the end.
- Boring, but Practical: They have the potential to defeat Smash by themselves. It just takes a lot of bullets. A lot of bullets.
- Butt-Monkey: In Heisei Generations FINAL, the Nebula Bugsters goons are swapped out for Guardians in the film's last act so that all the Riders get opponents they can beat since Bugsters are immune to attacks not backed by a Game Driver or weaponry associated with such.
- Combining Mecha: A whole group of them can combine into a larger mecha.
- Cool Helmet: The Guardians do have pretty cool helmet designs.
- Imperial Stormtrooper Marksmanship Academy: Whoever designed their targeting systems needs to be fired immediately. That said, their effectiveness against anyone but the protagonists (who Faust explicitly wanted alive for their own ends) imply their seemingly poor aim might just be for Pragmatic Villainy in those cases.
- Just a Machine: The heroes have no qualms destroying them, often in creative ways.
- Manchurian Agent: Namba Heavy Industries, as part of their pact with Faust, included a Villain Override function allowing Night Rogue and Blood Stalk to hijack them at a moment's notice.
- Mecha-Mooks: For every faction, with their ownership distinguished by their faceplates. Even Foundation X has some. All of them are produced and sold by Namba Heavy Industries, whose entire intent for starting the war was to sell weapons like them in the first place.
- Redshirt Army: They're basically government-issued Shocker Grunts.
- Villain Override: Namba Heavy Industries included one in their design, allowing Blood Stalk and Night Rogue to take control of them whenever they want.