Sometimes I boldly say, "Here I stand." Other times, as in a trick, I hide inside.................So who, or what, am I?

Question

Sometimes I boldly say, "Here I stand."
Other times, as in a trick, I hide inside.
I'm almost everywhere, it's not like I'm banned,
So I know you'll easily find me, in time.
If I were to say, "I'm in 'this' and not in 'that,'"
I'm sure you could find me in nothing flat.

So who, or what, am I?

Answer 1

The letter "i".