Adrenaline because it more or less fits all the clues.
Some rough areas (as requested by DrD)
I am the protector during a fright.
But you are not going to find me in any sight.
During fear andrenaline gives you energy so you can either fend of or flee the threat. It is invisible to the naked eye.
I am the protector during a fight.
But you will find me a little heavier than light.
Adrenaline enhances ones energies and reflexes fight are useful in a fight to hit and not get hit It makes one's heart beat more heavily so I guess its presence will be percieved as 'heavy.'
I am the protector during a flight.
But you will not find me flying like a kite
It helps gives you more energy to run faster from danger thus helping you get away and saving you from harm. Adrenaline being just a chemical cannot fly kites.
Your body will beat more as soon as you face me.
Adrenalin causes your heart to beat more when it increases in quantity in the blood stream.
Too much of my presence will kill you, and you will later see
In excess, adrenaline will cause you a heart attack, it also causes your pupils to dilate, I guess enhancing vision.