What is it that I look for?
The letter I. Some of the clues refer to homophones of this letter.
To many, it is most important.
To many, the eye is the most important sense organ. And "I", that is one's ego, is important to many, too.
To some, it is how they confirm.
"Aye" is a onfirmation.
To itself, it is about one fifth.
"I to itself", or i to the power of i, where i is the imaginary unit yields:
iⁱ = exp(i² π / 2) exp(−π / 2) ≈ 0.20788
— or about 1/5. Thanks, Sabre, for the leg up and Tweakimp for the "celebrity hint".
Telling you any more, I would just be giving you the answer.
Saying "I" would give the answer.
Hint 1: It is the one.
I is he Roman numeral for one and in some styles of handwriting it is also how an Arabic one is written.
Hint 2: It is a piece of history, lost in time, not confined to the past, present, or future.
I is in "history" and "time", but not in "past", "present" or "future".