Keisuke Nago (名護 啓介 Nago Keisuke) 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 it's top agent Takato Shiramine who never respected him. He is the current user of the Ixa System, 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!" (ファンガイア…その命、神に返しなさい Fangaia… Sono inochi, Kami ni kaeshinasai!).
Nago encounters Wataru when he is after a German fugitive, gaining the young man's praise for his confident exterior. When he learns Kiva has returned and that Shima had known all along, he demands permission to take Kiva out with the Ixa System. However, Shima assigns him the task of keeping Megumi safe, deciding to use that to have Kiva come to him and kill him. However, when Megumi threatens to file a restraining order, Nago uses Wataru to do the job for him instead. When he finally faces Kiva, Nago is about to become Ixa without permission until the Sheep Fangire kidnaps Megumi in the confusion. He is currently unaware that the person he is mentoring, Wataru, is the individual he is trying to kill. When encountering Kiva, Wataru manages to evade him, not engaging in any form of combat against the person he looks up to. However, when he was ultimately left with no choice, Wataru reluctantly transforms into Kiva and soundly defeats Ixa. Since then, Nago becomes mentally unstable to the point of attacking criminals without mercy while proclaiming that the Ixa System lost, not himself. Attempting to snap Nago back to his usual self, Shima presents him the Powerd Ixer, which is completed by the time the Rook fully reawakens. Unfortunately, Nago's encounter with the Seastar Fangire only drove Nago even more insane as he became obsessed with obtaining one of the Fangire's buttons as a memento of defeating him. Nago eventually got the button and added it to his collection, seemingly snapping back to reality.
After unsuccessfully trying to find out who Kiva really is by way of a fortune teller Kengo Eritate knew, Nago scolds Kengo for calling him 'old man' repeatedly (even though there is only a year's difference between them in age) and fights with Megumi against the Ladybug Fangire, with him and Kengo being kidnapped in the process before he shatters the Ladybug Fangire, forced to work with Kiva to destroy the Sabbat born from the remains. Since the Ladybug Fangire incident, Nago gains the admiration of Kengo who desires to be his apprentice to perfect his musical skills. While eavesdropping on a conversation between Megumi and Shima regarding his erratic behaviour, Nago finds out that a new upgrade being developed for the Ixa System, and is even more pleased that he will retain the Ixa System even after losing it briefly to Ryo Itoya, the Spider Fangire. Though he came close to being removed as Ixa's user, Nago succeeds in fooling Shima to let him stay as the user by pretending to admit he was flawed, becoming the first to use the upgraded Ixa System and its stronger form, Rising Ixa.
However, a man named Tanahashi uses his connections with the police to arrest all Wonderful Blue Sky organization members for the previous Ixa disregarding him in a Fangire attack and put him in the hospital, leaving him in a coma and unable to finish a painting of Maya he was working on. With Nago forced to run on the lamb, he encounters Jiro who him shows to Castle Doran's Time Door to give him the means to travel back in time. In 1986, he encounters Otoya and Yuri, learning that Otoya is the previous user of the Ixa System and tries to get the Ixa Knuckle from him. Nago then encounters Maya and quickly falls in love with her, though denies it, though he failed to her question about human love and gives her his good luck charm, his very first button as a bounty hunter, with a diamond-encrusted crown on one side. After learning that she was actually a Fangire, Nago manages to steal the Ixa Knuckle from Otoya and alter time by allowing Tanahashi to get away from the Cicada Fangire without injury. Once back in his time, after aiding Kiva in his fight against the Crab and Cicaida Fangires, Nago learns that the button he handed to Maya in the past now belongs to Wataru, who informs him that his mother gave it to him as a child. The revelation ultimately led Nago to learn that Wataru and Kiva are one and the same, seeing Kiva is not as much of a threat as he was told and deciding to take Wataru as an apprentice to ensure that while not caring about Kengo's feelings. Since then, Nago has been making regular visits to Wataru's house and joining him in his baths to advise the "right course" to take. However, Nago soon sees the consequences of his past actions when Kengo returns, trained by Shima to be a more suited user of the Ixa System when Nago gets injured in his fight against the Horsefly Fangire. Since losing the Ixa System, Nago made various attempts to either spite Kengo for taking over as Ixa or to help him, with Kengo thwarting each attempt at every turn.
Nago helps Wataru formally reveal his identity as Kiva to the rest of the organization, but also informs them that he is not a threat. However, he becomes stunned when Wataru reveals that he is half-Fangire. When Shima orders the members to eliminate Kiva, Nago pretends to follow orders and shoots near Kengo's (as Ixa) feet instead to delay his battle with Kiva. However, the battle becomes more intense when Taiga has Bishop awaken Wataru's Fangire nature until Nago calms him down. Although he was attacked by Kiva, he still trusts him, especially when he defends Kiva in front of Shima. After hearing that Wataru has become a recluse again, Nago desperately tries to get Wataru out, from coaxing him at the gate to even breaking into his house. However, when he calls Wataru to help Kengo, who is fighting the Seamoon Fangire and losing when Bishop arrives, Wataru informs him that this is his last battle. Realizing that Wataru is serious, Nago takes the Ixa Knuckle to decides to fight the Seamoon Fangire in Wataru's place until he is simultaneously attacked by both the Swallowtail Fangire and Saga. After being saved by Kengo, Nago manages to defeat the Swallowtail Fangire before Wataru returned as Kiva.
After Shima's death, Nago attempts to succeed his leader as the president of the Wonderful Blue Sky Organization, wishing to become stronger as a manifestation of Shima's will. He manages to defeat the Swallowtail Fangire again as a display of his newfound strength. However, while trying to protect Megumi from the revived Fangire, Bishop manages to seriously injure him. The blow was severe enough to cause Nago to gradually lose his eyesight. Due to this handicap, Nago considered quitting the Blue Sky Organization if not for Megumi intervening and encouraging him to continue as Ixa. He realizes his true feelings for Megumi as he finally defeats the Swallowtail Fangire for good and regains his eyesight. Days after the final battle, Nago marries Megumi prior to joining Wataru in a battle against a mysterious evil from the future.