Describing someone who has a great potential but is not aware of it

What's the best idiom/phrase/word to describe someone who has a great potential to do something while he/she is not aware of that potential. For this reason, (s)he underestimate her/his capabilities and asks others to do so for her/him.

Update:

There is a huge difference between my question and What would I call my friend's personality? In my case, they are not aware of their abilities but in the mentioned question, they lie about themselves. So, this is another question.

A person who does not utilize the talent/brains/charm they are perceived to have (by others) - whether due to insecurity, or other neuroses - is usually referred to as an "underachiever".

"Hidden" or "untapped" potential might fit the bill.

I'd call them unrealized, in saying they have "unrealized potential." Closely related is undeveloped, as in "undeveloped talents."

Perhaps with the most specificity there is unactualized, though it steps outside generic terms and into psychology.

There is also nascent and related terms if you wish to be optimistic.

unrealized - not made real or actual; not resulting in accomplishment, as a task or aim

unactualizied - To [have not] realize[d] one's full potential.

nascent - just coming into existence and beginning to display signs of future potential

The lack of self-awareness of future potential invokes for me the story of the ugly duckling.

There is a sense in which I'm not sure the idiom forms a great analogy in the human realm - the process of transformation into a swan was automatic rather than conscious, whereas untapped personal potential can usually only be achieved with effort and dedication. But for the legions of neurotically self-conscious teenagers who will - though they could not conceive of it - turn into fine adults and be a credit to their community, the "ugly duckling" concept is resonant. Often it refers to potential in the aesthetic sphere, but it is not limited to that.

http://idioms.thefreedictionary.com/an+ugly+duckling

The example sentence there is "The most successful company was last year's ugly duckling." This is an example of a non-aesthetic use, and achievement of previously hidden potential, which would serve for a person as well as a corporation.

Largely because of Disney's influence, such a person might be called a diamond in the rough, indicating a hidden value. This also implies that the value could be passed over by an unfocused observer, but the person so-referenced is not likely aware of the value either.