Evolt

"I'm the game maker. I derive the best strategy by considering every possibility. Everything is going as planned."

- Blood Stalk's speech to Night Rogue. "Even if I had never come to Earth, you people would've found yourselves causing another war soon enough. I remember telling you this before. Science always ends up causing destruction! Scientific advancement makes life more convenient, and people start to take things for granted. Before they know it, they will find themselves embroiled in war. That's the future...that science promises!"

- Evol's speech to Build in episode 35.

Blood Stalk is one of the leaders of the organization Faust and the primary antagonist of Kamen Rider Build.

His true identity is Evolt, a mysterious entity who annihilated the ancient civilization on Mars long ago using the power of the Pandora Box. He took over the body of Soichi Isurugi, owner of the cafe nascita and secretly the astronaut who discovered the Pandora Box 10 years ago, and used him to trigger the Skywall Disaster that separated Japan.

Blood Stalk/Evolt is voiced by Tetsuo Kanao.

Past
In the ancient past, Evolt used Pandora Box to create the Pandora Tower and annihilate the civilization of Mars with a massive distortion. To prevent him from moving on to other planets, Vernage, the Queen of Mars, fragmented Evolt's spirit from his body and sealed his power away in the Pandora Box.

Following the destruction of Mars, a fragment of Evolt's essence traveled to Earth with a Martian probe. After the probe was discovered, Evolt's essence entered the body of a security guard for the probe, Yuri Banjou and fused with her unborn son Ryuga. Ryuga was born a month later, containing the essence inside him though it did not influence his personality.

Ten years prior to the events of Build, Soichi Isurugi, an astronaut working with the Kiwami Project, discovered the Pandora Box while on Mars. Soichi unwittingly released Evolt's consciousness and possessed by him. At the ceremony back on Earth, under Evolt's control, Isurugi applied his hand to the Pandora Box, causing the Sky Wall Disaster.

"Soichi" was later arrested and placed in a mental hospital, along with his daughter, Misora Isurugi who was also exposed to the box and gained the ability to purify Fullbottles.

However, "Soichi" escaped about a month later. Seven years later, he was appointed as a special consultant for the Pandora Box. He also co-founded Faust with Gentoku Himuro.

Under Faust
Misora later joined Faust to purify the Fullbottles for them, though she was unware at first of their true purposes. When she eventually found out about Faust's intentions, she lost her will to continue working for them. "Soichi" then "saved" her and took her back to the cafe, where he convinced her to continue purifying the bottles for good.

"Soichi" later stole the Pandora Box panels from the Touto Institute of Advanced Matter Physics to ensure their confidence and plastered into one of nascita's basement walls. He also took along a Build Driver and some Fullbottles from Faust.

Dealing with Katsuragi
When Takumi Katsuragi attempted to quit Faust, Blood Stalk knocked him out and switched his clothes and face with Taro Satou, whom Stalk had previously killed by Stalk. Stalk then left Taro Satou in the alleyway and erased Takumi's memories before talking him back to Faust to experiment on him. On September 5th, Isurugi brought the amnesiac Katsuragi to a small alleyway. After Takumi woke up, Soichi renamed him Sento Kiryu and nurtured him into Kamen Rider Build so that, with Build fighting more monsters, Soichi would have reasons to have Misora purify more bottles.

Betrayal of Night Rogue
After Gentoku refused to reveal the location of the Pandora Box due to Stalk's treachery, Stalk began searching for the box on his own, eventually tracking it to the Namba Heavy Industries Integrated Science Laboratory. Stalk skirmished with both Sento and Ryuga and Night Rogue before teleporting away with the Box.

After Gentoku disbanded Faust to cover his tracks, he discovered that Juzaburo Namba had dropped Gentoku as a partner and instead partnered up with Blood Stalk.

His true identity was eventually discovered by Build and Ryuga after they defeated him in a fight and cancelled his transformation. His cover blown, Isurugi revealed to the two that Gentoku was Night Rogue and allowed them to keep the Green Panels and the Pandora Box.

Rebuilding Faust
Blood Stalk later rebuilt Faust within Hokuto and temporarily aligned with them, providing them with the resources they needed to create Kamen Riders. However, he later switched over to working with Seito when they invaded and conquered Hokuto due to most of Hokuto's forces being involved in the war with Touto.

After Seito lost a proxy battle with Touto and thus had to relinquish the Fullbottles and Pandora Box as per their agreement, Blood Stalk was summoned by Namba to kill Seito's Prime Minister Masakuni Mido and swap their faces so Namba could take control of Seito.

Becoming Evol
Stalk later attempted to get Sento and Ryuga to hand over the Evol Driver so he could regain his long lost power. The two were ready to deliver the Driver to Stalk, but chose not to and fought off Stalk with assistance from Kamen Rider Rogue. After being soundly defeated by the Riders, Stalk attempted to retreat but was chased down and knocked out of his transformation by Rogue. However, thanks to the actions of Nariaki Utsumi, Evolt managed to gain the Evol Driver and transform into Kamen Rider Evol. Overpowering Rogue, Evol prepared to finish Gentoku off but his attack was blocked by Gentoku's father Taizan.

Completing the Pandora Box
"The time is finally here. Taste the despair!"

- Evol as he constructs the Pandora Tower.

Eventually, Evol obtains all of the panels of the Pandora Box and uses them to assemble it and use it to create the Pandora Tower to destroy Earth. Horrified, Juzaburo Namba sends Nariaki Utsumi out to go stop him, but Evol simply shoots him away with the Nebulasteam Gun. He then laughs as the Pandora Tower is completed, declaring that "Earth is finished".

Evol then uses the Pandora Box to create the Evol Trigger, the device that allowed him to destroy Mars, but discovers that the body of Ryuga is not strong enough to use it. He is then ambushed by Vernage possessing Misora, who knocks him against a wall before teleporting away with Sento and the Pandora Box. She then explains to Sento, Kazumi and Gentoku who Evolt is and how he annihilated all life on Mars.

Back at Seito's capital, Evol claims to Namba and Utsumi that he created the Pandora Tower to showcase Namba's power to the world. Though unnerved, Namba goes along with Evol's explanation. Evol is later seen in Ryuga's body leading a Seito on Touto before Build steps in to fight him. Transforming into Evol Dragon, Evol and Build fight, and Evol reveals to Build that the whole point of Project Build was to create someone strong enough to harness the energy of the Evol Trigger. Discovering that Build has raised his power to superhuman levels, Evol abandons Ryuga as a host and tries to possess Sento, but Sento reveals this was his plan all along and attempts to kill both Evol and himself by rapidly raising his Hazard Level until he overclocks. However, Sento's plan fails and Evol succeeds in claiming Sento as a host, much to Ryuga's horror when he wakes up.

Possessing Sento
Ryuga attempts to transform into Kamen Rider Cross-Z to fight Evol Rabbit, but Evol reveals that he removed all of his alien DNA from Ryuga's body when he exited it and that without it, he cannot transform into a Kamen Rider. Kamen Rider Grease then tries to attack Evol, but Evol effortlessly beats him down and cancels his transformation before leaving. Meeting with Namba, Evol reveals his true origins to the war profiteer, but claims he only wants to regain his former power.

Evolt later arranges a meeting with Ryuga Banjou and Kazumi Sawatari, promising to return Sento to them if they hand over the Pandora Box. At the meeting spot, Evolt notices that he is being accompanied by Engine Bro's and Remocon Bro's, who were sent by Namba to keep an eye on him. After Ryuga and Kazumi arrive, Evolt reveals that he has no intention of letting Sento go, as revenge for him impeding the progress of his plans. He leaves the two to be fought by the two Kaisers, not considering them worthy of being taken care of by himself, though he does interfere in the battle when Grease starts to gain the upper hand.

However, after Kamen Rider Rogue appears and defeats Remocon Bro's and Engine Bro's, Evolt decides to enter the battle himself. Transforming into Evol Cobra, Evol engages Rogue and manages to overpower him after switching to Evol Rabbit. During the battle, unknown to Evol, Ryuga managed to snatch the Dragon Fullbottle off of Evol's belt. He then attacks Evol untransformed and somehow manages to inflict damage upon him. Shocked, Evol questions how he is able to do that without his DNA inside of him. Ryuga then uses the Dragon Fullbottle to transform into Cross-Z and then Cross-Z Magma before engaging Evol alongside Kamen Riders Grease and Rogue. The three combine their energies for a final attack against the Kamen Rider, but Evol manages to absorb their energies and use them to transform into his final form, Evol Black Hole, setting Sento free in the process.

Personality
Blood Stalk displays a disturbingly flippant demeanor, casually carrying out his duties as a Faust strategist and enforcer even when they involve committing atrocities. He generally acts more friendly when compared to Night Rogue, though he is just as sinister and ruthless as his ally, if not more so, turning a child into the Ice Smash just to show off his Steam Blade's ability to create Smash. He is also incredibly manipulative, and willing to play both his allies and enemies alike to achieve his goals.

Blood Stalk's cruel and sociopathic traits becomes much more highlighted as the series progresses, with him eventually being revealed to be Evolt, the entity that annihilated life on Mars. He becomes much more wrathful as Vernage and Sento continue to impede his plans, choosing to keep Sento around rather than erase his personality after he possesses him just so he can witness him harming his friends.

He also shows an apparent disdain for technology and innovation, claiming to Sento that science causes society becoming complacent and ultimately leads to war, though this may just be him trying to screw with Sento, as Evolt is shown advancing technology for his own purposes many times throughout the series.

Trivia

 * Initially, the true identity of Blood Stalk seemed to be Soichi after the reveal in Episode 13 and 14. However, in Episode 30, it was hinted that Soichi was possessed by some entity assumed to be Evolto. In Episode 33, said entity was confirmed as Evolto the whole time, including the time he was posing as Blood Stalk, making Evolto the one posing as Blood Stalk instead.
 * Evol is the second snake/cobra-themed Kamen Rider since Kamen Rider Ouja.