Low Devil!

Incredibly average protagonist, Ueda Yuuichi, has loved his childhood friend Shirotae Izumi since elementary school. But Yuuichi, unused to matters of the heart, lacks the courage to confess and has almost completely given up. However, when he meets Arismendi, a girl calling herself a demon, it rekindles his feelings for Izumi. Arismendi has come to the human world seeking a particular gem, and if Yuuichi helps her, she'll give him a magical item that lets him get friendly with any girl he wants... [Translation of synopsis from Getchu.com]