Adjective
astute - Ability to understand things quickly and clearly, and to use the knowledge well
From a very early age, Gail instinctively knew everything about business, and and after college she proved to be an astute business woman, and became a millionaire before the age of 30.
reticent - Describing someone who is inclined to keep their thoughts, feelings, or personal matters to themselves, not expressing them freely.

He was reticent about discussing his past, thinking that there were details that she might not react well too.