The Puppy Aptitude Test is a good predictor of future personality:
Volhard Puppy Aptitude Test
Did the breeder do health testing (bile acids, especially) on the parents before breeding them? Since this puppy is the "runt" and so quiet, I'd be concerned about its health. With liver shunts being so common in Yorkies (36 times more likely than all other breeds combined), you have to be very careful when buying a Yorkie puppy these days.