Mikoto Asuka is a third-year student at Suzaku High School who serves as the student council secretary and later vice-president. She is widely regarded as the school's brightest student and is known for her composed, intelligent demeanor. Underneath her calm and polite exterior, however, she harbors a sly, cautious nature and can display a sadistic streak when interrogating those she considers threats, as shown during her initial confrontation with the protagonist Ryu Yamada. She is fiercely loyal to the student council president, Haruma Yamazaki, acting as his personal bodyguard and carrying out his orders without hesitation.

As one of the seven witches, Asuka originally possessed the power of invisibility, which activated through a kiss. Prior to the main events of the series, she willingly allowed another witch, Tamaki, to take her power, thereby ceasing to be an active witch. Despite losing her magical ability, she remains physically capable, skilled in hand-to-hand combat and able to overpower opponents through sheer technique. Her role in the story is that of an early obstacle and later a supporting figure within the student council, working to maintain order at Suzaku High while also guarding the secrets of the witches. Her motivations are primarily driven by duty to the student council and loyalty to Haruma, though she occasionally reveals a mocking, unkind side toward those she deems unworthy. Asuka undergoes little personal change over the course of the narrative, remaining a constant, formidable presence who respects strength and cunning while using her own intelligence and authority to influence events behind the scenes.