Keisuke Nago is a 22-year-old elite member of the Wonderful Blue Sky Organization and former member of the 3WA (World Wide Wing Association), a loser at that time who left the organization due to his animosity towards its top agent Takato Shiramine who never respected him. He is the current user of the Ixa System in 2008, working as a bounty hunter by capturing criminals and taking a button from them as a memento. He originally had an intense hatred towards Kiva due to what he heard of the previous one, intending to kill him until he learns of Kiva's true identity and sees him as an ally to mankind. He also has a belief in "humanity's full potential", which gives him the impression of a superiority complex to some people with the ideal that he's always right. He has a prejudice for certain people like Wataru, as he believes they are everything his ideology of "humanity's full potential" is against. Because of this ideology, he has a distaste for music, as he feels it is a waste of that potential. It was due to his intolerance for sinners that Nago played a part in the suicide of his father years ago, exposing him to the media for taking a bribe. However, though he has a sense of righteousness, Nago's perspective on it is warped in his desire to purge the world of evil, refusing to accept the world in grey and going berserk whenever he loses a fight or is denied his quarry. Over time, most of these personality traits would eventually wane, although his sense of justice has not changed. When Nago transforms to Ixa, he first says, "Fangire… please Return that life to God!"

However, Ryo Itoya stole the Ixa Knucle from Nago and used it in an attempt to impress Megumi, only to be tricked by Megumi and the system was returned to Nago soon after. Megumi borrowed the System once to kill the Rook to finally finish what her mother started years ago. Due to Nago's incapacitation, Kengo Eritate was appointed the new primary user for Ixa, although his ability to use the system was quickly cast into doubt after his failure to destroy Kiva. After being defeated by Bishop, Nago relieves Kengo of the Ixa System, becoming its primary user once more. Although the Ixa System prototype was present in 1986, it had only Save Mode as Burst Mode was still in development. As such, technical problems were frequent like the armor shooting out steam, a constant beeping, a different tone of voice for the Ixa Belt, a bronze projection for the Ixa System's transformation and the availability of only one Fuestle: the Knuckle Fuestle. The Ixa prototype exhausts a qualified person's body considerably even if they are not human, as the System still managed to harm Jiro despite being a Wolfen, and it harmed Otoya to the point of total memory loss. Ixa has been continuously updated since 1986 and with the tenth model used until it is upgraded to the eleventh model with a new mode. Both Ver.X and Ver.XI have a golden projection of the Ixa System instead of the original's bronze color. A total of six Fuestles are available to the user as well as a new weapon: Ixa Calibur. Save Mode is the mode used by Ixa mostly in the year 1986 in one of its early prototypes referred as the "Proto Ixa". Burst Mode is the first of Ixa's modes to appear. As the name suggests, Burst Mode creates a burst of heat that damages everything in range when transforming from Save Mode. It uses the Ixa Calibur to perform Ixa Judgement and the Ixa Knuckle to perform Broken Fang. When performing the Ixa Judgement, a vision of the sun appears behind Ixa. Rising Ixa is the upgrade to the Ixa System, known as Intercept X Attacker Ver.XI. As it is the perfected version, Ixa's power is at maximum strength. This mode is accessed through the new Ixariser and the Ixa Knuckle Ver.XI. After inputting the code, Ixa's outer armor is ejected and the head crest changes once again. In this form, Ixa can perform the Final Rising Blast. The Ixa system would receive one final upgrade which allows the user to transform directly into Rising Ixa, using a blue projection system. The rising system was so powerful at first that it knocked Nago out of transformation the first time it was used from the massive recoil but eventually he learned how to overcome it and use it effectively.