Kyoudai: Shoujo Tantei to Yuurei Keikan no Kaiki Jikenbo

Two years ago, Akagi Kei's older brother Keiichi, who was a policeman with a bright future ahead of him, mysteriously vanished. The police eventually closed the case, but they never found any sign of what happened to her brother. After that, Kei was very busy as the oldest child in the house, especially since her mother worked nights, so it took her a while to recognize a strange new voice that was calling to her...