In-story, the character is the reincarnation of a young man killed by a gang of road pirates; they beat him, stabbed him, locked his body in a car and sent it off of a cliff. From this, we can assume it was a "vengeful spirit" of sorts... but it returns with a physical body, changed from its original form, and with some new supernatural abilities seemingly tied to its vehicle, the Turbo Interceptor. We see his "return" to our world via an electrical storm in the desert, materializing the driver [in black sleek armor and braces on his limbs] and the car itself...
But it's never explained where he came from, how he got the vehicle, or how he managed to come back in a new human form.
The producers said he was more of an "inter-dimensional traveler" as opposed to a ghost, but a lot went unexplained, and that makes it somewhat more complicated. Added to that, every time he kills one of those responsible for his death, he seems to get stronger, physically, as shown by the braces "disappearing" from one limb for each kill.
Is there any more information as to these details?