So mc was first year high school, he have older brother who was 1 year or maybe 2 year older than him

One day his brother went missing , his brother left a message in phone or something to not pursue the "rabbit"

mc also find a flash disk in his brother room and there is a document about code name like B-Rabbit or what, i remember it was animal code name

He then go to his brother school and his brother classmates seems act weird and know something, i think there is someone dead in the class and the code name rabbit has something with it. MC got some help from a girl which seems the brother classmate and when he investigate , someone try to harm mc to make him stop , and even try to kill him, like pushing him from the stairs.

It is not rabbit doubt and the art kinda shouju-ish