The person in the room that does not talk to anyone or talks very little. This person is usually very smart, polite, nice, and wise but will go unnoticed, but in contrast they can be extremely insulting and cold, if they wanted to be, as they don't care too much for nor about what people think.
The quietest kid in the room is really the biggest crook.
The quiet kid is possibly the nicest, politest, and smartest people you will ever meet. Likewise, they have the potential to be the most dangerous.