Takumi Katsuragi

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 is an infamous researcher at Touto Institute of Advanced Matter Physics and later Faust who created the Rider System and operated as the first before being altered into by Evolto. 
==History==
===Background===
Takumi Katsuragi was born the son of Shinobu and Kyoka Katsuragi. Since childhood, he was considered a brilliant prodigy. Katsuragi was present during the Sky Wall Disaster and learned his father was one of the main causes of it, though citing his reasons for "love and peace". Sometime after his father's apparent death, Katsuragi was employed at the Touto Institute of Advanced Matter Physics and revered as the talented "Devil's Scientist" for his research on the Pandora Box, the Nebula Gas, and the Sky Wall. While taking up his father's research on Project Build, he developed the Fullbottles and the Build Driver to filter out the Nebula Gas' raw power without the risk of becoming a Smash.
===Ex-Aid's World===
Katsuragi's prestige at the Touto Institute would be noticed by Kaisei Mogami, who invited him to assist in the construction of the Enigma and use Project Build to travel to a parallel world. Seeking a sample of Kamen Rider Ex-Aid's essence, Katsuragi as Build mistakenly intercepted Kamen Rider Genm while he was fighting through Gamedeus Cronus' Bugster Viruses. After accidentally eliminating one of Genm's lives, Build realized that he found the wrong person and promptly took his leave after identifying himself to Kuroto. Inspired by this encounter, Kuroto Dan created a Rider Gashat based on Kamen Rider Build.
In his second and last known trip, Build successfully found Emu Hojo and absorbed Ex-Aid's powers into a Fullbottle. Attempting to return back to his world, he was pursued by Parado who then spent the next two years in Build's world attempting to retrieve Emu's powers back. The reason why Katsuragi took Ex-Aid's powers is because it was needed to combat against Mogami in his pursuit to destroy their native world with the parallel world using the Bugster Virus, but was unable to complete enough Fullbottles at the time.
===Confronting Evolto===
Eventually, Gentoku Himuro caught wind of his human experimentation and had him arrested. Despite losing his position, Gentoku later recruited Katsuragi to Faust so he could continue his research. Within a few months, he advanced his research to create the Smash, the Transteam Gun, and the Hazard Trigger. Despite Faust's objectives, Katsuragi reiterated that Project Build's primary purpose was to combat the alien Evolto, whom he discovered was masquerading on Earth as Soichi Isurugi. Confronting the alien, Katsuragi's plan was foiled and his appearance was switched with the musician Taro Satou. Erasing his memories as well, Evolto smuggled Katsuragi out of Faust and made it appear as if "Sento Kiryu" was just an amnesiac man found in an alleyway nearby Nascita on a rainy day.
===The Amesiac Scientist===
To keep his research on Project Build hidden from Faust, Takumi entrusted the data to his mother, which was ultimately passed onto his new personality "Sento Kiryu", who was much more sympathetic and generous in contrast to his original self. The Evolto-possessed Isurugi indoctrinated Sento as Kamen Rider Build to fight Smashes and harvest their Nebula Gas essence into Fullbottles. Sento continued to pursue regaining his lost memories and would help prove Ryuga Banjo's innocence for Katsuragi's apparent murder.
After Sento defeated Night Rogue, he learned of his true identity as Takumi from Gentoku, which had overridden him with immense guilty due to Takumi's past war crimes. However, he ultimately managed to find the confidence, courage, and resolve to right Takumi's wrongs by becoming the hero the country needs, Kamen Rider Build.
===Return of the Devil's Scientist===
As Sento continued to fight Evolto and oppose the Japanese Civil War by preventing the Pandora Box from falling into the wrong hands, Evolto managed to temporarily take control over Sento's body. However after he was released, Sento's personality had been reverted back into Takumi's entirely. 
Shortly after regaining his memories, Takumi was horrified to discover that Evolto had reached his final form before retreating with the Kamen Riders. Back at Nacita, Takumi became overwhelmed by his new look and disdained from his nickname as the "Devil's Scientist". He apathetically treated Misora as a lab experiment, denies Ryuga Banjo's existence for having Evolto's DNA, and rejected Gentoku Himuro's apologies for using his research to cause war. While the Kamen Riders engaged Evol Black Hole, Takumi assumed Build RabbitRabbit Form but was unable to harness the Pandora Panel. He became horrified that Evolto killed the Washio brothers in front of him, despairing Takumi to see so many lives lost in front of him.
Takumi continued to assist the Riders in their efforts against Evolto. Although he behaved just like Sento, Takumi lacked the fundamental aspects of a hero, which prevented him from using the Genius Fullbottle that he finished from Sento's handiwork. Hearing of Sento's seemingly heroic deeds from Banjo, Takumi experienced a flow of memories coming into his mind. In their mental landscape, Takumi met Sento for the first time and asked him of he truly had believed in the power of trust and friendship. After Sento responded with a smile, Takumi told Sento to prove it to him by showing the future they could potentially build. The two join hands and Takumi assimilates himself with Sento, allowing the latter to regain control and successfully transform into Kamen Rider Build Genius Form.
===Guiding Sento Kiryu===
Even after his assimilation with Sento, Takumi continued to exist in Sento's psyche, only for the purpose of helping Sento recur the memories that he cannot remember himself, such as the Lost Bottles and the faking of Shinobu Katsuragi's death. After Shinobu's reappearance, Takumi began to doubt their father's intentions as he temporarily allied with Evolto. 
After Evolto's defeat, as Sento observed the new world that he created, Takumi bid farewell to Sento and disappeared from Sento's psyche permanently.
===NEW WORLD===
Due to the merging of the two worlds, it created its own version of Takumi Katsuragi separate from the already existing Sento Kiryu. Due to Killbus' re-activation of Pandora's Box from the White Pandora Panel, Takumi, his parents, along with several others who were exposed to Nebula Gas in the previous World of Build regained their memories of their time on the other world.
When Takumi regained his memories, he moved away from Shinobu, not forgiving him for siding with Evolto before and marked him to lead him making a weapon for war. He was later asked by Sento Kiryu to discuss their plans against Down Fall, since the two of them are the only ones who understand the properties of the Nebula Gas, as well as the newly created Phantom Liquid. After the war against Down Fall, Takumi was informed to have reconciled with Shinobu.
==Personality==
As a researcher, Katsuragi has no qualms going to any lengths to complete his research, even going so far as to use brutal methods such as human experimentation. Katsuragi himself claimed that he'll gladly sell his soul to devil, as long as science evolves even further. However, he is shown to still have a filial side towards his mother, although he does not seem to express it on the outside.
He also claims to be a pacifist when he was battling Genm. This was due to his reasons behind the Build System and his views towards its use after his identity was changed.
Eventually, it was revealed that his commitment to Faust's goals eventually wavered enough for Blood Stalk to consider him a traitor, and orchestrated the events that lead to his new identity and him gathering Fullbottles. However, it was revealed the reason why he created the Rider System in the first place was to defeat Evolto, and was appalled to hear Gentoku used it to instigate the Japanese Civil War.
Misora also pointed that Katsuragi, after his memories are restored, behaves just like Sento. But, the difference is that he is arrogant and dismissive to a fault, as he lacks the understanding of friendship of others, comparing that from a . Katsuragi also uses honorifcs in his sentences unlike Sento, who speaks using slang. While desperate in his objective to defeat Evolto, he doesn't seem to express any concern about killing Banjo because of his connection with the alien lifeform.
While existing as a figment in Sento's mind after the latter regained his original memories, Katsuragi often adopts a pessimistic and anti-heroic disposition, in contrast with his new persona's sense of justice and trust in those around him, in part stemming from his guilt over his actions in the past.
==Family==
New Shinobu Katsuragi.jpg|Shinobu Katsuragi - Father
Kyoka Katsuragi.png|Kyoka Katsuragi - Mother
Sento Kiryu Grease.png|Sento Kiryu - Alternate personality (separated in NEW WORLD: Grease)
==Powers and Abilities==
===Powers===
*Artificially Enhanced Physiology: As Katsuragi was able to become Build on several occasions, he likely underwent an experiment that injected him with a certain amount of Nebula Gas. After Blood Stalk changed his face and memories, leaving his body being experimented by Faust, he didn't mutate into a Smash and the gas retained his body without major side affects aside from Stalk's memory erasure.
===Abilities===
*Genius-Level Intellect: Infamously known as the Devil's Scientist, Katsuragi was a brilliant prodigy and revered as one of the best researchers at the Touto Institute of Advanced Matter Physics. Most of his intellect is passed onto Sento Kiryu.
*Master Scientist: Katsuragi has an advanced knowledge of physics.
*Master Engineer: Katsuragi has built the majority of Project Build's technology, which included the Build Driver, Transteam System, Sclash Driver, and Hazard Trigger.
* Bilingualism: Katsuragi speaks his native Japanese, and can possibly speak English as he is able to read files written in English entirely and perfectly translate it back to Japanese verbally.
===Weaknesses===
*Negative Emotions: Since the creation of the Genius Bottle, Katsuragi wasn't able to activate it since he did not have any trust whatsoever in anyone and the Fullbottle can only work the form based around the heroic traits, which his current persona had.
==Forms==
Katsuragi's Build Driver did not originally announce Best Matches, but was modified to do so by Sento Kiryu after Katsuragi's supposed death.
==Equipment==
===Devices===
*Build Driver - Transformation device.
*Fullbottles - Transformation trinkets.
*Fullbottle Holder - Carrying strap for Fullbottles.
*Hazard Trigger - Build's power up device, used to upgrade into a Hazard Form by attaching it to the Build Driver.
===Weapons===
Handheld Weapons
*Fullbottle Buster - Build's personal sword-like and cannon-like weapon, accessed by any form that uses the FullFull RabbitTank Bottle.
Suit-Equipped Weapons
*Rabbit Foot Shoes - Build's rabbit-like spring shoe accessed by any form that uses the Rabbit Fullbottle.
*Tank Roller Shoes - Build's tank-like tread shoe accessed by any form that uses the Tank Fullbottle.
*Muscle Glove - Build's gorilla-like gauntlet accessed by any form that uses the Gorilla Fullbottle.
*BLD Prism Glove - Build's diamond-like glove accessed by any form that uses the Diamond Fullbottle.
==Behind the Scenes==
===Portrayal===
Takumi Katsuragi is portrayed by . In Kamen Rider Ex-Aid media and Kamen Rider Zi-O, he was portrayed and voiced by who plays his current identity Sento Kiryu as so to hide his alternate identity away from viewers. As Kamen Rider Build, his suit actor is in his televised debut in the 44th episode of Kamen Rider Ex-Aid.
==Notes==
*Him being the founder of Faust and at the same time being called the Devil's Scientist alludes to in which the organization he founded is named after; a scholar who made a pact to the Devil exchanging his soul for unlimited knowledge and worldly pleasures.
*In Kamen Rider Heisei Generations Final: Build & Ex-Aid with Legend Riders, it is revealed that Takumi was the man behind Build's mask in the final episodes of Kamen Rider Ex-Aid and in the post-credit scene of Kamen Rider Ex-Aid: True Ending. Furthermore, the film retroactively establishes that the Build Driver would not announce Best Matches until Sento took over, leading to a slight chronological error for marketing purposes. 
*Despite Takumi the character himself having transformed into Build several times, his actor Yukiaki Kiyama has never transformed into Build on-screen, except for one failed attempt in Episode 34.
==Appearances==
==References==
==Navigation==
ru:Такуми Кацураги
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