Jun is a successful supermodel in Tokyo when the story begins. She is found by a woman, Lan Asuka who forces her to confront a werewolf-like monster called a Devil-beast. As it tears into Jun, her own powers awaken within her and she becomes a Devil-beast herself. She defeats the beast with her new powers and reverts back to her human form. Asuka then tells her that she is a member of a military unit known as the Human Alliance whose purpose is to hunt down and kill all those who are affected with Devil Beast Syndrome. Asuka gives Jun an ultimatum; if Jun doesn't hunt for them, she will be hunted by them. Jun has no choice but to obey Asuka from that point on.
Jun is a very quiet and timid person who seems to avoid most social interactions, with two exceptions. She is generally open with her manager and outright friendly with an aspiring teen model named Kazumi. As the series progresses she becomes much more independent. She is the second human with devil beast syndrome capable of retaining her ability to reason.
In the manga Jun is very different she is strong and outgoing from the start. She is also a gym teacher and not a model in the manga.