Final Fantasy VII Crisis Core takes place before the events of Final Fantasy VII, therefore introducing many characters who do not appear in Final Fantasy VII. This includes Cissnei, a friend of Zacks and a member of the Turks. With her apparent disappearance, you may be wondering what happens to Cissnei in Crisis Core and why she isn't in Final Fantasy VII.
Why isn't Cissnei in Final Fantasy VII?
Cissnei does not directly appear in Final Fantasy VII. However, she seems to be briefly mentioned or teased when Cloud goes to Gongaga. Zack's mother mentions that a woman who was Zack's friend came to see her. This most likely refers to Cissnei, as Aerith was Zack's other female friend. This cannot be Aerith, though, as she never left Midgar or came to Gongaga.
Zack's mother doesn't give a name, but this likely would not matter as Cissnei confirms that it isn't her real name to Zack. If Cissnei gave a fake name to Zack's mom, it wouldn't connect any dots for Cloud, even if he knew of Cissnei beforehand.
The most likely outcome for Cissnei after the events of Final Fantasy VII Crisis Core is that she is still searching for Zack. Despite Shinra reporting his death, Cissnei believes that her friend is alive. Add in reports of Cloud, a former SOLDIER who appears similar to Zack and even uses the Buster Sword, and she will likely have conflicted feelings as to Zack's demise.
She could also be considered a loose end by Shinra and perhaps hiding from the corporation for fear they will come after her. She could easily have changed her name and disappeared to stay alive. Unfortunately, we do not officially know what happens to Cissnei during Final Fantasy VII Crisis Core and beyond, so this is all speculation.