Pocha Musume wa Koakuma Move ga Yamerarenai

Tsukiyono Mayoi, 16: A second year at Oujibaru High School, and a part of the school's Library Committee. Whenever she's in front of the kohai she likes, Amagi Haru, she acts like a clingy 'little devil' character. It's not something she does by choice, but rather, by necessity. Her unappealing figure and low confidence makes her insecure, and forces her to put on an act in order to talk with the guy she likes. And so begins the story of a chubby girl that just can't seem to stop acting like a little devil character.

(Source: Kill Time Communication, translated)