Momo

On her sixteenth birthday, the unlucky, unmotivated and good-for-nothing Odagiri Yume saves the high-classed and lovely Momo, who claims to be the Devil. Yume confesses her wish to become special and Momo grants it: she makes Yume the representative of the earth! From now on her sixteenth birthday until her eighteenth, Yume has to give Momo seven good reasons as to why she the earth shouldn’t be destroyed. The countdown for the destruction of the world has begun and only Momo can stop it.

(Source: Starry Heaven)

Included one-shots:
Volume 1: Kimi no Sekai no Sukuikata (キミの世界の救い方, How to Save the World)
Volume 7: 17 O'Clocks (17o'clocks)
Komai doesn't really interact with her classmates, she likes having space for breathing. But what happens when the most popular boy in her school approaches her?