Kyoudai: Shoujo Tantei to Yuurei Keikan no Kaiki Jikenbo

Two years ago, a promising police officer named Keiichi disappeared without a trace, and the case was eventually abandoned. His younger sister, Akagi Kei, became the primary caretaker at home while their mother worked night shifts. She eventually noticed a strange voice calling out to her—her brother's skeletal ghost. He appears disoriented, claiming to have come from a dark forest filled with vengeful spirits. Though he was always protective, he now seeks to help others and safeguard his sister from danger. Kei is now attending a prestigious high school where supernatural elements are beginning to affect the students. Together, they use their combined abilities to uncover the mysteries troubling their classmates. Kei hopes to one day uncover the truth behind her brother's death.