Izumi Aomi, also known as Aomi Izumi, is a student at Miyazaka High School, attending class 1-5 which is in the year next to the main protagonist's. She is a teenage girl with short, neck-length blonde hair that she has dyed, and purple eyes. Her appearance is often described as that of a delinquent or a bad girl, as she wears her skirt as short as possible and has a habit of smoking cigarettes, substituting them with lollipops when she cannot smoke. It is rumored she often skips class to associate with other troublemakers, though behind this rebellious exterior lies an exceptionally high level of intelligence.

Aomi possesses a sharp and curious mind, which leads her to investigate the strange occurrences happening around her school. She becomes aware of a system that erases the memories of both students and faculty whenever an anomaly occurs, and she becomes convinced that the student council is hiding significant secrets. Her investigation leads her to uncover files related to the military, a discovery that attracts the attention of the teacher Kurosu. Rather than punish her, Kurosu invites her to join the student council, telling her that if she is so eager to throw her life away, she may as well do it for them. Despite initial opposition from another council member, Gekkou, Aomi secures her place by proving her academic merit, matching Gekkou's scores in their exams and surpassing him with a perfect score in home economics.

Her motivations are complex and deeply tied to her own mortality. She is willing to place herself in dangerous situations because doing so makes her feel alive and like she is living life to the fullest. This is compounded by a tragic personal discovery: she has a brain tumor that, by the prediction of modern medicine, will end her life by the time she turns eighteen. This knowledge gives her actions a sense of urgency and a fatalistic bravery.

As her role within the student council expands, Aomi joins them on a summer training camp designed to improve their combat and teamwork abilities. She begins to learn magic, and after a mission to hunt an ancient dragon where a colleague is seriously injured protecting her, she is overwhelmed by guilt. Determined never to be a burden again, she asks the powerful magician Saitohimea to alter her brain structure to allow her to use magic more effectively. It is during this procedure that Himea discovers the tumor, a secret Aomi forces her to keep. Later developments in the story reveal that the mass in her brain is in fact not a simple tumor. It is a curse, and a god of prophecy had hidden its power there to escape the notice of other powerful beings. This curse eventually causes her to become possessed by a character named Haruka, who seeks to use her body as a vessel to kill Gekkou.