Non-Player Characters (or NPCs for short) are the collective term used by the Afterlife War Front to refer to the majority of the student population. They have many characteristics similar to a real person, but they are distinguishable by their ability to easily recognize persons who have recently arrived in the afterlife. NPCs do not age and have no souls, and anything weird that they see is supposed to be normal (at least, they think it's normal).
It was once speculated by Yuri that any member from SSS would disappear if they followed the lives of the NPC students (such as studying, which explains why all of the members would rather fail in their exams), but the belief was later proven false when the truth was known that the members would only disappear once they've achieved their goals or fulfill their dreams they didn't managed to do before their deaths. For an example is related to Iwasawa, the founder of Girls Dead Monster and a member of SSS who passed on after her achievement to perform her last song in front of the audience with her guitar was accomplished.
During the later episodes, it is also known that the NPCs are susceptible to manipulation that will transform them into dark shadows for use as a weapon. Since this has happened, this event is part of a automated system created by the nameless Programmer and is activated when the amount of love in the world reaches a certain limit and the world must be reset (in the view of The Programmer). The shadows created have a single motivation, which is to engulf and transform humans into NPCs.
Like Takamatsu, Yuri falls victim to one of the attacks of the shadows. During this period, she experiences being an NPC, which is one where a person simply experiences life as a student without any major negative experiences. When questioning The AI in Episode 12, Yuri is told that there is one human in the NPC population, the nameless programmer.
It is currently unknown what would happen if someone were to indeed kill an NPC, or what would happen. In Episode 6, it was shown that Kanade had killed the NPC's that Naoi hypnotised, not suffering any consequences of her actions for killing them. In Heaven's Door, it was shown that Yusa had murdered many male NPC students, but like Kanade, she suffered no consequences of her actions.
Why Yuri would take extreme precautions for anyone in the SSS not to kill the NPCs remains a mystery.