Satsuki Shinomiya was a student in the 9th Grade Class 3 of Yomiyama North Middle School in 1987. Unbeknownst to her classmates, however, she was actually deceased, returned to school by the Calamity as the Extra.
Little is know about Satsuki's role in the Calamity of 1987, although she did spend three months of summer vacation staying with Teruya Sakaki's family, along with her friends "Ni," Mitarai. Yagisawa, and Higuchi. She was apparently very close friends with Teruya, and it can be inferred she was not killed by the Calamity again in 1987, as the Curse ran the full term that year.
After term, Satsuki returned to death as all other Extras had before, ceasing to exist in any official records, including a picture taken at the Lakeshore Manor. However, she still showed in Chibiki's records, so Mei was able to learn her identity in 1998.
While it's never expressly made clear the nature of the relationship between Teruya and Satsuki, Teruya at least cared very deeply for his classmate, and was plagued with guilt over abandoning her and his other classmates during the 1987 Calamity. In an attempt to protect her and his other friends, Teruya convinced his father to allow Satsuki and the others to spend the summer vacation months at their retreat outside Yomiyama, the Lakeshore Manor. While there Satsuki posed for a picture alongside Teruya and her other friends, though after the Calamity had run its course she ceased to exist and was removed from the image. Despite this, Teruya chose to hold onto the photo, despite not knowing why it held such strong emotions for him.
Part of Teruya's mind knew someone important to him had been ripped from his life, but due to the Curse he couldn't remember who she'd been. Because of this, Teruya became fixated on the idea of death, and becoming one with previous victims of the Calamity, culminating in an attempted suicide on May 27 of 1998. When Teruya was killed in a confrontation with his sister, Tsukiho, he saw Satsuki just before death, and his last words were her name.