Playful Lullaby

Fun times don't last, memories fade, but bond can never be severed. Reve was leading a fairly lazy but normal life until one summer vacation. His childhood friend Arya suddenly invited him to go on an adventure by solving mysteries written on an old notebook Arya found. As they try to solve those weird mysteries, Reve started to discover interesting but odd facts about their village. Where did these mysteries even came from and who wrote these on the notebook Arya found?