From MangaHelpers:

Aku Maya is a demon, come to do some hands-on practice in the human world. She's living at a boarding house, attending school as a human girl, and keeping her demon nature secret from her close friends Sui, Sawa, and Arata. Maya's supposed to be increasing and collecting the negative energy of the humans around her with her 'evil checker' phone, but she doesn't seem to be able to take much pleasure in it. Besides that, what would her friends say if they found out she was a demon? [tethysdust]