Ryoma Sengoku

"My brilliance... and research... are the only things of value. They are the truth of this world!"

- Ryoma Sengoku while being confronted by Kaito

Ryoma Sengoku is a researcher for the Yggdrasill Corporation and the true main antagonist of Kamen Rider Gaim. He works under Takatora Kureshima, the head of Yggdrasill's research and development branch though unbeknownst to him, Ryoma is secretly plotting to overthrow him.

Ryoma is also the creator of the Sengoku Drivers, making him responsible for a large portion of the plot.

Past
Ryoma was among the children raised in the Zawame Child Care Facility. There, he was conditioned to work for Yggdrasill.

Ryoma would go on to become a scientist for Yggdrasill as well as meet Takatora Kureshima. Together, the two created and developed the Sengoku Drivers.

Ryoma later received a report from his subordinate Yoko of an image appearing to be the Overlord Inves Demushu. Ryoma neglected to tell about this existence due to Takatora's philosophy of putting humanity first.

Beat Riders Saga
Ryoma informed Takatora about the new Driver model, which Takatora offered to test the belt. Ryoma later attempted to get the 7 prototype Sengoku Drivers in the Helheim forest. Though Takatora didn't want any outsiders to be in the forest, Ryoma told Takatora to not destroy his precious work. During the Inves breakout of this event as they run rampant to Zawame City, Ryoma aids Gaim through the use of the Yggdrasill's tower through the use of a cloaking mechanism and beam weaponry.

Ryoma later created the Yomotsuheguri Lockseed in an attempt to harness more power from Helheim. However, after discovered that the Yomotsuheguri Lockseed drained the lifeforce of its user, Ryoma abandoned it. Ryoma also noted that the first Lockseed he created was dubbed the Ringo Lockseed which gives a potent amount of power that allows users to manipulate Cracks at will. However, the Ringo Lockseed overwhelmed its user by spouting Helheim vines from the Ride Wear. Ryoma also worked on a successor prototype of the Suika Lockseed dubbed the "Watermelon Lockseed" because the Suika Lockseeds were hard to come by and had to recharge. This Lockseed was scrapped due to the fact that the output of the Watermelon Lockseed was inferior to the newly created Energy Lockseeds and that the Watermelon Lockseed drains the stamina of the user much more quickly.

Yggdrasill Saga
Ryoma had Takatora test the Genesis Driver, noting its impressive strength. Ryoma later watched Sigurd kill Ryoji Hase. Ryoma then introduced himself to Kouta and Kaito and asked for their permission to continue using them to complete his quest for power, but the two escape. Ryoma later discovered that the Genesis Core slot and the Lemon Energy Lockseed is stolen and pondered why someone would take an incomplete core slot prototype.

Ryoma used the data received from Kouta's Sengoku Driver to improve the mass-production models. He also examined the data from Jimber Lemon Gaim and Marika's fight and decided to test how his Genesis Driver would do on Kouta. Transforming in Kamen Rider Duke, Ryoma easily defeated both Jimber Lemon Gaim and Baront.

Ryoma later attempts to trick Kaito into finding the Over Lords without the equipment to combat them. Ryoma also apologized to Kaito for what Yggdrasill did to his family, by buying and destroying the construction factory Kaito's father owned.

Later, when Yggdrasill suffered several attacks from an unknown Armored Rider capable of producing Helheim plants, Ryoma was informed of the appearance of the Rider. Ryoma provided Takatora with a prototype version of Suika Lockseed, as a back-up plan for if Takatora lost his Genesis Driver and needed to use his Sengoku Driver. Upon learning the Armored Rider has the ability to create Cracks at will, Ryoma became concerned and realized what Lockseed the Armored Rider used, although only Takatora thought Ryoma might have been thinking about something to do with the orphanage under the direction of Takatora's father, Amagi, in which Ryoma used to belong to, despite Takatora being unaware of Ryoma's origin.

Upon discovering that the Armored Rider was actually Touka Akatsuki, Ryoma revealed his past in Yggdrasill's child care facility. Ryoma then kills her in order to reobtain the Ringo Lockseed.

Ryoma created a prototype of the Dragon Fruits Energy Lockseed and gave it to Alfred. When the destruction of the Lockseed caused Alfred to become a pseudo Overlord, Ryoma sent Yoko to give Kaito the Ringo Lockseed to destroy Alfred. Kaito succeeded in destroying Alfred, but at the cost of the Ringo Lockseed.

Later, when a Crack appears on a bridge in Zawame City, Takatora sent Ryoma down to the other side of the Crack in the Helheim Forest. Ryoma later explains Kouta what Project Ark and Yggdrasill plans to reduce humanity 1 billion in order to eliminate the potential 6 billion Inves, as well as how the Sengoku Driver will help the remainder of humanity as it converts Helheim's fruits into Lockseeds and enables the wearer to absorb the nutrients safely without side effects. As the Crack closes, Kouta steals Yoko's Peach Energy Lockseed and rushes to HQ to destroy the Scalar System.

Helheim & Over Lord Saga
Ryoma later apologized to Mitsuzane for giving Kouta classified information. Ryoma later realizes that Kouta is being supported by someone else when Kouta uses the Kachidoki Lockseed and tells Mitchy to find out who it is. Mitsuzane later informs him and along with the other Yggdrasill employees of creatures even more dangerous to humanity than the Inves. Realizing that their chance for power is dwindling, Ryoma provides Sid with another Cherry Energy Lockseed, so as to give Sid a chance to kill Kouta.

DJ Sagara later and Ryoma tells him about how the history of Helheim and how it has affected human civilization. Sagara then disappears after instructing them to find the Overlords.

Seeing that the Sengoku Driver isn't a match against an Overlord, Ryoma let's go of Kaito from his group. However, Yoko gives Kaito a Genesis Driver replacement to provide him with another chance. When figuring out how to find the Over Lords, Mitsuzane tells them that they can easily find the Over Lords if they adhere to his plans.

After Takatora discovered the Over Lords, Ryoma attempts to dispose of Takatora and transforms into New Generation Rider Duke. Ryoma defeats Takatora, with Sid throwing him off a cliff. They later go to Yggdrasill's headquarters and erase all of Takatora's records.

However, Sid destroys all of the equipment around the tree, causing the Headquarter's Crack to disappear and close. Ryoma then tells Kouta, Kaito, and Mitsuzane to stop Sid from getting the Forbidden Fruit. When Demushu attacks Zawame City, Duke cuts off all communication in the city save the forces fighting Demushu. When Helheim's plants begin to absorb Yggdrasill Tower, Duke abandons Marika and the other Yggdrasill employees and escapes in a rocket, causing him to lose Marika's loyalty.

Forbidden Fruit Saga
Ryoma later attends a holographic conference between the executives of Yggdrasill's international branches. The executives threaten to arrest him due to the Japanese branch's responsibility disaster, but Ryoma uses a bug in the executives' phones to expose Project Ark and Yggdrasill's plans to cull the population to the world, sparking a global panic.

Ryoma then goes back to Zawame. He meets Kouta in Team Gaim's garage and mocks him about Takatora fate, provoking a fight between the two. However Armored Riders Knuckle and Baron appear and ambush him. Ryoma surrenders and reveals to the alliance of a secret entrance to Yggdrasill Tower before leaving them.

While Oren and Hideyasu fight the Inves, Ryoma and Kouta enter Yggdrasill Tower. However, they are confronted by Shinmugurun. Kouta fights the Overlord, but isn't able to defeat it. Duke steps in and takes down Shinmugurun easily. However, he doesn't kill him to see to collect data on the Gaim Kiwami Arms. After the battle is over, he makes a thought that it will be interesting to collect more data about Kouta's new power.

Arriving on the site where the kidnapped citizens are, Ryoma leaves Gaim to fight Redyue. Ryoma then reboots the Master Intelligence System to access records prior the Inves invasion. Upon discovering Rosyuo's name while analyzing the footage, he theorizes that the White Over Lord is the one holding the Forbidden Fruit. Ryoma later secretly follows Kouta and Kaito to gather data of Rosyuo's power in Helheim. Despite the Forbidden Fruit from Rosyuo expiring, Ryoma discovers that Rosyuo planted a second Forbidden Fruit in Mai. Ryoma returns to Zawame and forms an alliance with Mitsuzane to extract the fruit. They bring Mai to a hospital where Ryoma examines her body. Sagara then appears, explains his purpose to them before disappearing again. Ryoma informs Mitchy that he can remove the fruit from Mai, but states that Kouta would interfere and gives Mitchy the Yomotsuheguri Lockseed to defeat Kouta with. Ryoma also warns Mitsuzane the consequences of using it before he leaves.

Later, Mitsuzane returns and Ryoma states that he did not expect Mitsuzane to survive. Ryoma reveals to Mitchy that the Forbidden Fruit had fused with Mai's heart and, in order dissect it, Ryoma had to kill her. Enraged, Mitsuzane tries to attack him, but Ryoma disables his Genesis Driver. However, the Fruit turns into an apparition of Mai and disappears.

Ryoma attempts to locate where to fruit is, but he is confronted by Kaito and Yoko, who sought to avenge Mai. Ryoma disables their Genesis Drivers, forcing Kaito to use his Sengoku Driver instead. Because the Sengoku Driver is weaker than the Genesis Driver, Duke defeats him. However, Kaito consumes a Helheim Fruit, changing him into an Over Lord Inves called Lord Baron. Ryoma attempts to fight Lord Baron but Lord baron easily defeats him. Before Lord Baron can kill him, Ryoma tells Kaito that he won't survive long and falls off the edge the off the building they're on, causing him to plummet to his death.

Movie War Full Throttle
"It's been a long time, my friend."

- Mecha Ryoma's greeting to the Kureshima brothers

However, he returns in the film Movie War Full Throttle when he's revived by the mechanical lifeform Megahex as Mecha Ryoma Sengoku. Mecha Ryoma equips the Dragon Fruits Energy Lockseed and transforms into Kamen Rider Duke. He attacks the two Kureshima brothers.

Later, Mecha Ryoma fights Takatora in a decisive battle. Takatora uses his Musou Saber to stab through Ryoma's Genetic Ride Wear and kill him. Mecha Ryoma said his last words to Takatora before dying with his Genesis Driver being taken by Takatora to allow him to transform into Kamen Rider Zangetsu Shin.

Trivia

 * He's named after the Japanese historical figure Sakamoto Ryōma.