Hiiro was inspired to become a doctor by his father Haima, the director of Seito University Hospital despite the man's own clumsiness. By the age of 19, Hiiro was a medical student at Seito University, and in a relationship with fellow student Saki Momose. Despite Hiiro's cold (and as a result, hurtful) personality, Saki deeply cared for him, going so far as to leave him so that he could prioritize his studies. When Zero Day occurred, Saki became infected by the Bugster virus and created Graphite, a Bugster so powerful that not even Taiga Hanaya (as Proto Snipe) was able to defeat him. By the time Hiiro learned of Saki's condition, Graphite had emerged as a complete Bugster. Saki only had enough time to ask that Hiiro become the best doctor in the world before vanishing into data.
Hiiro was strongly affected by Saki's death. Knowing his cold attitude worsened Saki's condition, he blamed Taiga for her death in an attempt to suppress his guilt. He also began eating pastries in between operations (despite his dislike for dessert) as a memento to Saki, who often offered him sweets to refuel.
Sometime in between the Bugster outbreaks, Hiiro saved the lives of Mizuki Nishikikoji and Satsuki Ogimachi, both of whom would assist him as nurses, vowing to be the best aid for the best doctor in the world.
Hiiro was working for a prestigious hospital in the United States before being called back by his father to handle the new Bugster outbreak. Given the Gamer Driver and the Taddle Quest Gashat, he had just arrived in Japan when a second candidate, pediatrics intern Emu Hojo, had been chosen to join the CR as Kamen Rider Ex-Aid. Because of their different points of view, the apathetic Hiiro would always find himself at odds with Emu's idealism and care for patients, responding to the latter's presence with "No thank you". As more Kamen Riders emerge, imcluding Lexx as Proto Brave who buts heads with Hiiro over the right to be the True Brave more bugsters appear including Taddle Quest. But both are defeated and the fience of the victim steps in to become Brave without surgery and win the battle handily, reconizing a natural prodogy Hiiro takes Asumi under his wing.
In time, Hiiro would come to respect Emu's position as a Rider, but not enough to actually like Emu, as he had other issues to attend to in the interest of saving lives with the help of his aides, Satsuki, Mizuki and Asami. That was, until his late girfriend Saki Momose, in reality the Graphite Bugster shapeshifting into her. This would incidentally drive a wedge between the surgeon and his protege, as he resumed his relationship with his resurrected love like she had never died. This dedication gets tested, however when Power Rider Extreme raids CR and easily kills Kuroto, Kiriya and Kiro, with him surviving through timely intervention by his girlfriend, her fully committing to being Saki by then.
It was also in the aftermath of said raid, while trying to process what just happened when Saki is confirmed to be the Graphite Bugster trying to spy for Parad, which he forgives her for after seeing her earnest attempt at redemption. As the weeks wear on, though, it's clear that he's trying to make the best of a worsening situation which included the wide release of Power Rider Go and Emu and Nico becoming Evolt's minions. He also takes on the case of Yamato Jin, working with him in an attempt to save his sister Akira from the virus, starting with The Ayakashi and then Hokutei, pulling out all the stops to do so. It culminates in taking on the latter, clearly outmatched despite having help from Saki and the wolves while also at Level X.
The heroes throw everything to take him down and keeping him down, eventually resorting to a two-way sacrifice play when it's clear not much is actually working. It works, at the heavy cost of Saki's life and Hiiro's ability to transform, leaving him broken and powerless once more. At this point, the only saving grace for him was that Hokutei went down with Saki, and the relief of Lexx's return to CR, while also learning that Emu had recovered and is now operating a clinic with Blecker and Taiga's help.
Hiiro is intelligent and capable, yet cold and arrogant. Hiiro firmly opposes personal involvement with his patients, believing that personal feelings will interfere with his performance. Hiiro has a liking for desserts, mainly cakes or pies, which he claims is to increase his energy in between operations. Hiiro treats his fighting with Bugsters as an operation. He has a habitual quirk of raising his hands in the position most surgeons use when they are about to operate on a patient, and refers to the Gashacon Sword as his "scalpel". He often also speaks in a mannerism akin to his civilian job after transforming. (Ex. "Commencing Bugster removal operation"). He will cancel his transformation if he deems the situation too risky (primarily when his Rider Gauge is almost depleted). Whenever he encounters someone he doesn't like or doesn't feel belongs in the situation, he will respond to their presence by saying "No thank you".
Name: Hiiro Kagami
Age: 24
Birthday: [Redacted] 1992
Gender: Female