Kane is somewhat of an anomaly in Hell, he's human, whereas all of Hell's other inhabitants (excluding demons) are furries.
Though most of Kane's life is unknown, what is known is that he was alive around the time of Jack and Central; it is also hinted at that he may have been the one to kill Central. Kane was apparently a weapons-grade microbiologist. Several scientific papers and machinery belonging to Kane seem to describe how to create artificial viruses. Kane also seemed to have somewhat of an obsession with immortality, having worked and perfected a machine that could actually bring dead beings back to life.
He may have been behind the "Reworks War".
In death, Kane came to embody the Sin of Envy; what he did to obtain this rank is unknown. Instead of tormenting the denizens of Hell like most of the other Sins, Kane prefers to work behind the scenes, manipulating others and plotting. Currently Kane is amassing an army of Rework zombies and attempting to get the other Sins to align with him, so far allying with Emily (Pride) and Bob and Lisa (Gluttony). What he and his allies are planning to do is, once again, unknown.
In his Sin form, Kane resembles something along the lines of a primate, being almost completely covered in thick brown hair. Kane also possesses jagged fangs and what appear to be claws in his Sin form.