There are two types of heroes in this world:There are everyday heroes like Rainbow Dash and Twilight Sparkle. Masters of their elements, the two revel in their lives. Others praise them, look up to them, aim to be like them. They are true heroes.
Then there are powerful heroes like Princess Celestia and Luna. Masters of Day and Night, the world would not be able to function without them. The land would fall into chaos without their guidance and all would suffer. They are like Atlas in that the world rests upon their shoulders. They are true heroes.
I am neither of these things.
I keep silent watch over all whom I can. I do my best to prevent harm to all bodies I can manage. Day or night, I go many sleepless nights to make sure that those around me do not have to.
My face is hidden always. It is best for all that my identity remain a mystery, even for myself. If my identity were known, then others might try to prevent me from my own duties. They would want to protect me when I must protect them.
It is because of these things that I am not a hero.
It is because of these things that I am the Mysterious Mare-Do-Well.