It must be the
apostrophe-sign, like in "I'm" or "I've"
Explanation follows...:
First of all, the apostrophe is used throughout the whole riddle, so it is probably the clue.
I could be there, yet I'll stay hidden.
Hidden I'll stay, but you'll need me.
Need me you will, so I may appear.
Appear I may, I don't decide.
Decide I don't, my master does.
The apostrophe may appear or may not. The writer decides if to include the apostrophe.
I'm a word, I tell you words.
I've a task, you know my task.
I'm short, I shorten your path.
I've the answer, now tell me it!
The apostrophe makes the statement shorter. It is no word, but it "tells" you words.
Second guess, based on the first one:
abbreviation