Akuma no Kagi to Shoujo no Raison d'etre悪魔の鍵と少女のレゾンデートル ; Akuma no Kagi to Shoujo no Raison d'Être ; Girl's Raison d'être

One day, Natsuto found a loli near his house and it turns out she's a Holy Loli that is in a mission to eradicate the demons that escape from the Chaos Realms and as well as find the Key that can summon demons. Source: (Kisu KMS)