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He is a cop chosen to use the G3 System to battle the Lords, whom his superiors labeled as "the Unknown", and keep them from murdering people, Hikawa is an earnest, straight forward, and honorable man, though slightly naive and clumsy. Because he was a regular human, the Lords usually ignore him; but sometimes are surprised that a normal human being can wield such power.
One day, while on patrol, Hikawa witnessed the Akatsuki being held by the El of the Sea, taking a boat to investigate and save all but one of its passengers. Because of his heroic role in the incident, which showed his skills in a crisis, Hikawa was assigned to the G3 Unit.
Hikawa took the G3 System on its first run during the Jaguar Lord case, resulting with a feral Agito damaging the Suit. But overtime, while aiding Agito, Hikawa managed to piece together the nature of the Lords' victims. But when Hojo took his place as the G3's user, believing he got the position for being a hero, Hikawa got transferred to criminal investigations for a short while until he proved his skills by anticipating the Zebra Lords' next target, resuming his role in the G3 squad while decking Hojo when he attempted to capture Agito.
Soon after stronger Unknown, and Hojo's Victory One System, made themselves known, Hikawa was given the upgraded and G3-X Suit that is better suited for Unknown Combat and "perfect" in every way. However, the perfection was the flaw as the AI would momentarily take over an impure wearer, as seen when the suit acted on Hikawa's deepest desires when it took control. After consulting her mentor, Ozawa installs a special inhibitor chip to regress the AI.
Hikawa finally learned the true identity of Agito in episode 39 when he cancels his transformation in front of him after being knocked by Another Agito, and Hikawa was visibly shocked to learn Agito was Shouichi, much to his disbelief this makes him lose his cool (even getting in a short argument with Ozawa and Omuro) and expresses disappointment at the revelation and can't accept it at first because the true identity of Agito he admired was far from what he expected. This caused Hikawa to be hesitant on meeting Shouichi. However, their combined effort to defeat Skelos Glaucus caused them to continue their usual relationship, And Hikawa become more respectful towards Shouichi.
After Hojo took over the G3 Unit, Hikawa was transferred back to Kagaki-ken. However, Hikawa couldn't leave just yet, so he and Ozawa take back the G3 Unit. Donning the G3-X one final time, Hikawa aids Agito and Gills in finishing off the Lords for good. By the next year, Hikawa leaves the G3 Unit and returns to being a normal officer of the law.
Kamen Rider Agito: Heaven's Door
There were a lot of killings happening with the recent one being a nude woman found impaled on the top of Tokyo Tower. An Agito was sighted at the scene. While Hikawa was investigating, the Agito and a dog-type Unknown attacked him. Though Exceed Gills and Another Agito appeared to his aid but they were easily getting defeated. Luckily Agito/Shouichi Tsugami finally shown up. The real identity of this Agito was Shouichi Maguichi, who was crazed after being blamed for cheating on the programs for using a hidden camera during his childhood. Wanting to cure his sick mind he worshipped the Dog Orphnoch/Dog Lord and was told to feed on the despair of the public. After Agito and the others killed the Dog Lord/Orphnoch, Maguichi/Agito transformed into Mirage Agito out of rage. In a fight with Kamen Riders Agito and Gills, despite gaining the upper hand he was defeated by the strategized use of the Shining Caliber and a Shining Rider Kick. Maguichi was then arrested and was given an unspecified treatment.