Kazuya Aoi is the second child of Orie Aoi, Ryuuichi Aoi and younger brother of the late Kazuha Aoi, the hero of the 8th Nova Clash, and the grandson of Gengo Aoi, the chief scientist of the Chevalier and the creator of the Pandora, Limiters, and Stigmata. During his mother's pregnancy complications arose as a result of his stigmatic body, forcing Gengo to transfer Kazuya from his mother's womb to his aunt Cassandra's in order to save his life. Kazuya was then raised by Cassandra Aoi until he was two years old.
Kazuya is physically unimpressive, with short black hair and brown eyes. Like all Limiters seen so far, he has a slender build. He's usually seen in the traditional West Genetics uniform but dresses in casual clothes and the Limiter combat uniform when the situation warrants it. He is seen as "cute" by a few of the girls in school. Recently, after tapping into his potential from extreme stress during the 13th Nova Clash, Kazuya underwent a transformation into his Nova nature in which his eyes momentarily became like his grandmother's, which had vertical slits over each pupil, showing that he has become more inhuman and more like the Nova.
Kazuya is a kind boy who doesn't like to bad-mouth others and cares deeply for his friends, qualities that eventually get the aphephobic Satellizer L. Bridget (who is shunned by others due to her past and rumours of her ruthless behaviours) to warm up to him. He is serious when Satella becomes injured in fights and tries to stop them on his own.
Kazuya hates all unnecessary combat, especially between allies. Despite his kindness, or perhaps because of it, he reacts extremely aggressively when others act cruel and callous towards each other. This is seen when he Freezes the thugs assaulting Rana and Kaho Hiiragi and knocks one of them out with one punch, and again when he strikes Ingrid across the face to make her truly understand her friends' final words. He is also rather intelligent and perceptive, recognizing a group of thugs for who they were after a few minutes of interacting with them, and being able to adeptly read many situations he is confronted with. He still retains a certain amount of naiveté especially when it comes to personal/physical relationships like he has with Satella. His kindness comes with a high amount of what can be termed "chivalry".
He is always polite and respectful to everyone, especially females. He tries his best to respect Satella's feelings and personal space, even suppressing his own physical desires on numerous occasions and berating himself for having them in the first place. He also seeks to remain formal and respectful with Rana Linchen, despite the latter's' efforts to be informal and intimate with him, as seen when he tried to keep the massage he was giving her for her new arms professional and platonic. These traits have made him allies and friends with almost everyone he has met, or at least earned their respect.
However, after the revelation of his origin and his connection to the Legendary Pandora, Kazuya begins to have a mental break down, becoming angry and abrasive to everyone around him, including Satella who he rejects out of fear and anger. He has shown to become sullen and withdrawn, not saying anything when he and the "Legendary Pandora" are escorted through town, and even averting his eyes from Satella when they almost look at each other. Kazuya eventually overcomes his emotional problems and returns to his normal self, thanks in part to Satellizer's assistance.