Hina Asanagi is a university student who seeks out the help of the Millennium Research Institute, specifically Ichiro Umoregi, also known as Akuma-kun, and Mephisto III. Her primary role in the story begins when she approaches them to investigate a disturbing and mysterious event: the deaths of two of her friends, who both perished on the exact same date and at the same time. This incident serves as a catalyst, drawing the main characters into a new case.

In terms of appearance, Hina is depicted as a young woman with brown hair that falls to her neck and gray eyes. Her personality, as inferred from her actions, suggests a sense of responsibility and care for those close to her, as she is noted to watch over a friend named Ichika. The danger that persistently lurks around her and her social circle highlights her vulnerability as an ordinary human caught in extraordinary, supernatural circumstances. Her motivation is driven by a desire for answers and justice for her deceased friends, pushing her to involve herself with the demon-fighting Akuma-kun. Her key relationships include her deceased friends, whose unexplained deaths are the central mystery, and Ichika, a friend she feels compelled to protect. Hina's role is not as a combatant but as a client and a witness, grounding the supernatural narrative in very real human tragedy and concern.