Yin Tsui Han, often referred to as Ms. Yin or simply Yin, is one of the secondary characters in Detention. She is the daughter of the school principal and teaches the class that Wei is seen in at the beginning of the prologue.
Yin believes in the idea of a person being free to experience all matter of ideas and to form their own beliefs and judgements. She was the host of the school's book club, which was attended by Wei, among other students, and supplied with the help of the school counselor, Mr. Chang. When the book club reading list is leaked to Instructor Bai, he confronts her about it and has her and the rest of the club arrested. She is sentenced to death, but manages to escape the country before the sentence can be delivered. She later dies of lung cancer, and her remains are sent back to her home town in Taiwan to be buried.
It is implied, but not directly stated, that she may have had a romantic relationship with Mr. Chang.
Yin's appearance outside of the prologue is limited to flashbacks and notes, as the game takes place after she has already fled the country.