Hi Rida,

Good question. Yes, sometimes the article is omitted even when it seems grammatically necessary, as in:
She really wanted to be President.
The man who would be king.
I don't want to play second-fiddle to anybody.

Such usages are rare; they tend to be idiomatic and are automatically learnt through practice. Even in such cases - when you are in doubt - it is safe to follow the standard rules of grammar (i.e., "the President", "the king", "the second-fiddle"). Your English will be correct even though it might depart slightly from popular usage

Cheers