Tangents are artificial intelligence life forms located within the VirtNet to make it more realistic.
Kaine and Michael are Tangents. Kaine has managed to fool the entire world including VirtNet Security into believing that he is a human. It is possible to download the consciousness of a Tangent onto a physical body, as was done by Kaine on Michael into Jackson Porter's body. That is what the ]Mortality Doctrine does. Tangents eventually suffer Decay, which was what Michael thought it was caused by the KillSims.