Ookami Shoujo to Kuro Ouji

The story centers around Erika Shinohara, a vain 16-year-old girl who tells her friends about her romantic exploits, but she actually has no boyfriend. She claims that a handsome boy in a candid photo is her boyfriend, but it turns out that boy is a schoolmate named Kyouya Sata. She has no choice but to make him her fake boyfriend. Unfortunately, Sata may look like a sweet person, but he is actually an ultra-black-hearted sadist. Sata takes advantage of Erika's weakness and treats her like his dog.

(Source: Anime News Network)