Dark horse

One who keeps himself or his aims hidden. Keeping things secret, or concealed even seemingly pointless secrets they can be referred to as a dark horse when the interest, relationship, or action is revealed. Perhaps surprising, unexpected, even vaguely scandalous.

"Did you hear Dick won a mountain biking race?"
"Wow! What a dark horse, I thought he was a couch potato!!!"
"Trent has a new hot younger girlfriend, they were keeping it secret because he's her dad's friend"
"What a dark horse!!!"