I'm speaking of bare particles. "Heroes" is maybe too pathetic, but blocks or "bricks" would be OK since everything is made of them despite their
being non-observable. Why are they non-observable? Because they are
non-interacting particles or particles "before interaction". Inaccessible, for short.
It is
relativistic and gauge-invariant because the bare electrons, positrons, and photons are such.
Parameters 
and 
are bare electron mass and charge and
the term 
is how bare
particles interact. Of course, bare particles have spin and other
quantum numbers.
You may wonder how do we physicists know all that
if the bare particles are non-observable (and why do they interact if
they are non-interacting particles)?
Good questions. Very
intelligent! The answer is - due to our physical insights. You know,
insight is the ability to see the invisible, to penetrate mentally into
the unknown, to figure out everything correctly from small, rare, and
distorted pieces of a whole picture. Factually we, from long distances
(from low-energy experiments with physical particles), penetrated to the
end - to the point 
where bare particles live. Thus we
insightful nailed the bare particle properties and their interaction
laws correctly despite their hiding from us.
And yes, the bare
non-interacting particles do interact and even self-interact. It is they
who permanently do this hard work. At a first, naive, glance these
statements are inconsistent, but no. It is a kind of duality in physics.
This duality is not much advertised because the bare particles are
really modest bricks.
