The Puppet (aka Marionette or Slenderman) is a mentally unstable animatronic marionette that resides with the rest of the Fazbear crew in Freddy Fazbear's Pizza. Though they are very insane, they know more about the lore of the past and the pizzeria than they seem to. In fact, they know about as much as Golden Freddy does. It was once believed that they went mad due to isolation in their box for a long period of time. However, in a recent FNAF 2 let's play, the cutscenes before and after the videos revealed that it was really because they used most of their abilities and sanity to give life to the animatronics and Paper Pals.
As stated above, they have the ability to give life which they use to bring life to the toilet, a toaster that created I Am Bread, and cure the hack that happens whenever animatronics play Popgoes (Albeit, since this was a theory created by themself, there is a possibility that they simply made themself the hero. Face it, we've all had those fantasies.)
They were a victim of the Fazbear Massacre and became a phantom. Thus, they fell under Springtrap's control and worked to kill the remaining Fazbear residents.