Mad-Eye Moody

'Yeah, if we get any hint that Potter's been mistreated in any way, you'll have us to answer to,' Moody said.
Uncle Vernon swelled ominously. His sense of outrage seemed to outweigh even his fear of this bunch of oddballs.
'Are you threatening me, sir?' He said, so loudly that passersby actually turned to stare.
'Yes I am,' said Mad-Eye, who seemed rather pleased that Vernon had grasped this fact so quickly.
'And do I look like the kind of man who can be intimidated?' barked Uncle Vernon.
'Well...' said Moody, pushing back his bowler hat to reveal his sinisterly revolving magical eye. Uncle Vernon lept backward in horror and collided painfully with a luggage trolley. 'Yes, I'd have to say you do, Dursley.'

-Chapter 38, The Second War Begins, Harry Potter and the Order of the Phoenix by JK Rowling

from 2005-ish, revisited spring 2006