Usotsuki Yuriko no Eikou

Synopsis

She wanted to be popular, but she had nothing to offer... Yuriko, an ordinary junior-high girl, lied on her first day at school about being engaged to a popular, good-looking classmate. She thought she'll be found out soon enough, but for some reason, the boy took advantage of her lie and pretended to be her fiancée! As she piled lies upon lies on top of lies, her selfish act became a way to help others...!? (Source: Kodansha, translated)