Oninagi

Oninagi (オニナギ-鬼薙-) is a manga series by Akira Ishida. It was serialized in Manga Time Kirara Forward. It was published in English by Yen Press from April 21, 2009 to October 28, 2014, as well as in eBook format from April 22, 2014 to October 28, 2014.

Plot
Typical schoolgirl Nanami has just started high school and can't wait to spend as much time as possible with her first crush. But her blissful high school days take an unexpected turn when the sword-wielding demon slayer, Tomotaka, confronts her, claiming Nanami has an evil power within her that must be destroyed.