Sin and Death are the two guardians of the Gates of Hell. Sin was born from Lucifer's head at the moment of his rebellion; this means that Sin begins with rebellion against just authority. Death was born as a result of an incestuous relationship between Sin and Satan.
They are the keepers of the Gates of Hell. When Sin opens the gate, it can never be shut again. The mother and son together build the road from Hell to Earth, so that while they are causing trouble with all the creatures there, the devils from Hell can easily travel to Earth--and the condemned souls from Earth will easily slide down to Hell.
Two other allegorical figures that influence Satan are Rumour and Discord (both of whom should be obvious in their roles).