Lilu

Ten years after the Battle of Hogwarts, Slytherin freshman Leelou Ashcroft steps into the castle with the secret of her mother's forbidden magic. She meets five partners who also have secrets, and they hunt down a gray robe "Echo" among Quidditch, Potions class and secret passages. Seven years of laughter and danger intertwined, friendship is fire, and the truth is hidden behind the first door they opened - inside the door there was an entire forgotten era and an unfinished sentence. "The echo wasn't a ghost—it was someone who lived so hard that the stone remembered it for her."