Chika Idol wo Kage de Sasaeru Ore

A relationship between Natsukawa, a flashy gal-type classmate, and Kageyama, a plain and unassuming boy. Despite appearing to have no connection, Kageyama secretly receives a message from Natsukawa: “Meet me at our usual karaoke spot after school.” He joins her for a private karaoke session, where he reveals something she had asked him to prepare.

(Source: Natalie, translated)