"Viper the Stealth," a Master Assassin from the Demon Lord's Army, has Infiltrated the Hero's Party

Story:

Tasked by the Demon Lord to "win the Hero's trust," he ruthlessly takes down even his own demon forces, believing "to fool your enemy, you must first fool your friends." But for Nanami, the female Hero whose extraordinary abilities have kept her from a normal life, Viper doesn't just earn her trust—he captures her heart!