The Sibyl System is a collection of human brains which possess all aspects of their former human's personality, memories and thoughts. All are criminally asymptomatic, possessing neither a Psycho-Pass nor a Crime Coefficient; thus making them near-invincible in their power over society insofar that they can neither be recognized nor categorized by any method known to Sibyl itself.
The hive-mind is a collective of personalities, but only one mind at a time may inhabit the body of Joshu Kasei. It may be speculated that Kasei's body is either robotic, android or biomechanical since it seems to be able to regenerate itself with ease, as seen in Perfect World.
In Psycho-Pass, only two personalities are seen to occupy Kasei's body: Kozaburo Toma and Misako Togane. The rest of Sibyl's members remain anonymous.