To answer your second question, Soo-hee is played by the new Korean actor Han-byeol Park. Han-byeol Park uploaded a photo of his school days, which caused a sensation because it was very similar to Gianna Jun's appearance. You can go to her post bar. She also has other film and television works.
Sue has a lesbian plot with Jane Xing and has vowed to be with Jane Xing forever. After her death, she possessed a fat girl in order to get close to Jane. At first she didn't want to kill Jane, but she still hoped to be with her in this way. However, Zhenxing was extremely frightened when facing the fat girl possessed by Sue (as you can see in the film, she didn't believe that the fat girl was Sue at all), which led to Sue's hatred from love, killing Zhenxing and taking her away. At the end of the film, Su, who became a ghost, killed Zhenxing as she wished and disappeared from school with the soul of Zhenxing.
To answer your third question: the last girl who plays the cello is a freshman. While sorting things in the dormitory, she unconsciously hummed a song that Sue used to dance, and said to herself, "What is this song? It seems that she has heard it before. " Then in the photo, Sue gave a strange smile. The girl was specially arranged by the director to make the finishing touch for the film. You can imagine her as the reincarnation of Sue. She has memories of past lives in this life, so she hummed that song. The strange smile on the photo indicates that another tragic cycle is about to begin.
You can ask me if you have any questions.