Reasoning Equations Carefully

Three years ago at an amusement park, a 14-year-old detective named Zhou Yiran uncovered a man's crime of seeking wealth and murder. A few years later, he entered the second year of high school. On campus, he met Dai Xinyi. In addition, there were four people in the class: Xu Chuyi, the top student class president, Xu Chuyi, the top class deputy class leader, Jiang Zhizhi, the president of the Mystery Research Society, Xie Yunzhe, the mystery novel writer, and Liu Xubai, a sports master and a scumbag in cultural classes. The six of them were good friends when they were at school or investigating crimes. However, on the day of their graduation trip, Dai Xinyi was kidnapped by a person. Who was that person?