Disbelief

Harry thought he'd heard wrongly - his expression said the same, so Tonks repeated;

"I'm the Half-Blood Prince."

Lupin seemed embarassed for Tonks, yet slightly amused; he exited the room.

"But - you're not..." Harry didn't know how to say this. "I thought the Half-Blood Prince would have to be - well..."

"But I am a half-blood," Tonks replied.

Harry bit his lip. "I thought a Prince would have to be... male."

Tonks looked as uncomfortable as Harry felt. "I'm a Metamorphmagus, remember?" she mumbled. "I needed this disguise..."

It made sense, Harry thought, as the awkward silence intensified... why she was called by her surname... why she prefered to have short hair...

"You didn't honestly think anyone sane would make me an Auror?!" laughed Tonks. Harry understood her reasoning.

"What exactly does a Half-Blood Prince... do?" he said, finally.

Tonks sighed. "Well..."