Ayase Kamiyugi is a sixteen-year-old girl of half Japanese and half German heritage who serves as one of the central figures in the narrative. She is easily recognized by her long, waist-length blonde hair and green eyes, traits she inherited from her mother. Her past is defined by a single, traumatic event: the brutal murder of her parents by angels from the White World during the Black Point incident. This tragedy completely upends her formerly privileged life as the heir to the wealthy Kamiyugi family and sets her on a dark and vengeful path.

Driven solely by a need for retribution, Ayase suppresses nearly all other emotions, which gives her a permanent, cold, and intimidating demeanor. She is often self-serious and straightforward, a personality trait that her partner frequently exploits to tease her. Despite her hardened exterior, she is still a teenager, and on rare occasions, a side more appropriate for a girl her age slips through, revealing an occasionally clumsy or flustered nature. Her overwhelming hatred makes her an anti-heroine, as her quest for vengeance is so absolute that she is willing to cause massive destruction, even to the point of nearly committing genocide against a specific type of Z/X, the very creatures from another world.

To achieve her goal of eradicating the angels, Ayase seeks out power from the Black World and captures a powerful black panther Z/X named Quadruped Victor Zieger. Their relationship is not one of friendship but a calculated, transactional pact built on two conditions. First, Ayase will provide Zieger with resources using her card device, and in return, he will assist her in her revenge. Second, and more darkly, Ayase agrees to surrender her own life to Zieger the moment her revenge is complete. Zieger is a warlike creature who enjoys combat and is often dissatisfied with only hunting angels, preferring to fight any worthy opponent. He frequently mocks Ayase for her single-mindedness, creating a unique partnership between a vengeful girl and her bloodthirsty, sarcastic familiar.

In the story, Ayase operates primarily as a lone operative on a crusade against the White World, earning her the feared moniker of Angel Killer. Her role becomes more integrated with the main cast when her pursuit of an angel named Fierte leads her into conflict with the protagonist, Asuka Tennoji. Her initial hostility toward the pacifistic Asuka gradually evolves as their paths continue to cross. While not a traditional hero, she is one of the most active and plot-significant characters, with her personal quest for revenge against the angel Gamuviel forming a major storyline that eventually dovetails with the larger conflict. Her development is measured; she begins to temper her all-consuming hatred, learning to accept help and even forming a grudging alliance with her former enemies. She eventually succeeds in defeating Gamuviel, but the discovery that the source of her parents' death is tied to a larger threat involving the Devil's Mask artifact gives her a new reason to fight beyond simple vengeance.

Ayase is a highly capable combatant with strategic intelligence and formidable power in battle. As a player, she possesses high resource technology and capacity, allowing her to effectively summon and maintain her Z/X in the human world. Her primary ability in battle is tied to her partner Zieger, with whom she can perform an Ignition Overdrive to unleash devastating attacks. Her signature techniques often involve eliminating multiple opponents on the battlefield, and she can even temporarily manifest the power of the ancestral god Leviathan in critical moments. Later in the series, her power evolves to the point where she can achieve a Vow Armament form with Zieger and, in alternate timelines, she is capable of transforming into the high-ranking angel Raphael, the Angel of Healing.