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Yua Yaiba is the technical advisor for A.I.M.S. and one of the main characters of Kamen Rider Zero-One. Despite serving A.I.M.S., Yua is actually affiliated with the global megacorporation ZAIA Enterprises and is only on loan to A.I.M.S. from Zaia. She seems interested in escalating tensions between humans and HumaGears for unknown reasons. She has the power to transform into Kamen Rider Valkyrie with her Shot Riser.
Yua Yaiba was among the A.I.M.S. operatives deployed in response to the Berotha Magia's rampage alongside Isamu Fuwa and several soldiers. While Zero-One engaged the Magia, Yua and Isamu fought the Trilobite Magia it summoned.
Yua later visited a sushi shop Hiden Intelligence's new CEO Aruto Hiden and his HumaGear secretary Izu were at to observe a sushi chef HumaGear, Nigiro Ikkan. Yua ordered a slice of sushi from the HumaGear and, after it was prepared, tasted it and remarked on how good it was. However, Yaiba was then notified about a Magia attack and left to confront the Neohi Magia. During the fight, Yua revealed her Kamen Rider form, Kamen Rider Valkyrie, and destroyed the Trilobite Magia spawned by it. After the fight, Yua reported to Zaia Enterprise's CEO, Gai Amatsu, revealing that she had planted a bug on Nigiro Ikkan that would allow them to spy on Hiden Intelligence.
After MetsubouJinrai.NET deployed Little Assassin on its first mission, Yaiba abducted the HumaGear while Jin was engaging the Kamen Riders and took it to a secret location, where she reprogrammed it so the lights in its eyes were blue, the sign of a normal HumaGear, instead of red, the sign of a hacked HumaGear. Yaiba then set the HumaGear loose again and filmed it with her phone, making sure to capture in the video that its eyes were blue, before sending the footage to the media so they would report on it and cause a scare about HumaGears going rogue, which would sink Hiden Intelligence's stock.
Later on, Yaiba assisted Zero-One in defeating the Mammoth Magia, only to recover the Zetsumerise Key it dropped to deliver to Zaia. Yaiba later met with the CEO of Zaia Japan, Gai Amatsu, and handed him the key.
Yua then went to check up on the production of the Giger units at A.I.M.S.'s base that they had ordered from Zaia. However, A.I.M.S. was then attacked by MetsubouJinrai.NET, who in the process hacked one of the Gigers and injured Fuwa, forcing him to be rushed to the hospital. MetsubouJinrai.NET then remotely activated the Giger they had hacked and programmed it to go hospital to hospital hacking all of the HumaGears within them, forcing Hiden to shut down all of the HumaGears working at hospitals to prevent them from being hacked. To get Hiden to reactivate the HumaGears so Fuwa's HumaGear doctor can operate on him, Yua gives the Breaking Mammoth Key to Aruto, which allows him to transform into Kamen Rider Zero-One Breaking Mammoth and destroy the Giger.
After the incident, Yua goes to Hiden Intelligence to talk with Izu, only to be confronted by Izu about the fake video she filmed of Little Assassin after Izu deduced the person who filmed the video was her. Caught off guard, Yua unconvincingly denies filming it and says it wasn't her before leaving.
Yua later reports to Gai Amatsu, who expresses surprise at Yua handing over the Mammoth key to Hiden Intelligence before ordering her to obtain another Zetsumerise Key. Following this, Yua and the rest of A.I.M.S. were dispatched to provide security to the filming of a movie produced by Hiden starring a HumaGear actor. After MetsubouJinrai.NET attempted to assault the production but were driven off, Fuwa learned of the faked video Yua had sent to the media and went to confront her about it, reminding her that A.I.M.S.'s mission was to keep watch over the HumaGears and not to take down Hiden. However, Yua replies that the latter is her true mission from Zaia and informs him that there will come a time when they will be on opposite sides before heading off.
Yua later took part in AIMS's raid on MetsubouJinrai.NET's base at Daybreak Town, which was supported by Hiden Intelligence. Despite the raid seeing the death of Horobi, AIMS was forced to retreat after too many of their members were injured. Later on, when Jin took charge of MetsubouJinrai.NET's forces and led a crusade on the city, Yua and Fuwa responded and assumed their Kamen Rider forms to fight off Jin's army, but were overpowered by Jin's hacked Gigers. However, Aruto Hiden then arrived and transformed into Kamen Rider Zero-One Shining Assault Hopper before destroying all of MetsubouJinrai.NET's forces, including Jin.
Following the destruction of MetsubouJinrai.NET, Yua met with Fuwa in A.I.M.S. base where they had recovered and taken the body of Horobi. Yua later reported MetsubouJinrai.NET's destruction to Gai Amatsu, who stated that MetsubouJinrai.NET would never be gone as long as HumaGears existed. Gai then revealed himself as the one behind the formation of MetsubouJinrai.NET, having caused its creation when he fed the Ark knowledge of humanity's propensity for ceaseless wars. A shocked Yua then accuses Gai of using A.I.M.S. and MetsubouJinrai.NET as his pawns when she and Fuwa were risking their lives to stop MetsubouJinrai.NET. Gai calmly responds by telling her the real battle had not yet begun and that he would like her to continue serving as his right-hand.
Return to Zaia Enterprise
Yua was later called back to Zaia from A.I.M.S. She oversaw the first round of the competition between Hiden's HumaGears and Zaia's ZaiaSpec involving floral arrangement.
Yua was later confronted in a parking garage by Fuwa, who demanded to know if she had repaired Horobi and why she had done it. Yua declined to answer and instead asked Fuwa why he suspected her, to which Fuwa recalled her earlier warning that she would betray him someday. The two then assume their Kamen Rider forms and fight. Valkyrie initially gains the advantage by attacking with ranged attacks but Vulcan manages to overpower and defeat her by getting in close. However, Gai Amatsu then shows up and transforms into Kamen Rider Thouser before beating down and knocking Isamu out of his transformation. Gai then instructs Yua to go with him and the two depart.
In contrast to her partner Isamu Fuwa, Yua is very cool and logical, rarely making decisions based upon emotion. She is polite in conversation and affable to everyone she meets. She does not share Fuwa's hatred of HumaGears, though she still views them as little more than tools that can be reprogrammed and not as actual living beings and comrades like Aruto or Izu do.
In truth, she is actually a spy from Zaia Enterprise tasked with causing Hiden's stock to plummet so Zaia can acquire them. Despite her ulterior motives for being part of A.I.M.S., Yua is on good terms with Fuwa and the rest of A.I.M.S. She sees Fuwa as a friend and cares for his wellbeing, showing concern whenever he gets hurt or overexerts herself. She also seemingly hopes to be on good terms with Izu and even expresses remorse that her goals will eventually divert her from Fuwa.
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