Alastor Moody
Alastor is an old man and an Auror. He has a magical eye that can see through things and rolls around in the back of his head. He also has a peg leg. During his year of teaching he taught the kids how to defend themselves against the Unforgivable curses. Though he wound up being a fake at the end of the year. And indeed was Barty Crouch Jr. who was there only to try and get the dark lord back to power...he succeeded.
Now, the real Moody, who spent that year locked up in his trunk is now a member of the order (actually he was a member the first time Voldemort was in reign) and is also a member of Harry's Advanced Guard.