Complixities are an umbrella term for the immortal beings who watch over the Complex. Many are seen as gods, but many more work behind the scenes to keep everything running smoothly.
They live, work and occasionally play within the Hub and branches.
Complexitites aren't naturally immortal. They have taken the initiative of GU1 and use nanobots who continously repair their bodies. This can be turned off at anytime in the NB control room located at the top of the Hub.