-- Alastor Moody, quite frequently
Retired Auror, considered one of the best Dark Wizard catchers the Ministry has ever had. Moody came on staff for a year at Hogwarts, but spent it at the bottom of a magical trunk, unbeknownst to anyone but the imposter who put him there (GF). Moody is a member of the Order of the Phoenix.
Moody’s favorite expression was “Constant vigilance!”
Mad-Eye is noted for his paranoia, particularly in regard to food and drink, but also wand safety; this last may be related to his own missing leg (GF, OP3), but that is mere speculation. Moody takes command of any operation he’s associated with. He’s brusque and efficient and anticipates the moves of the Death Eaters.
He organized the rescue of Harry Potter from Privet Drive for the last time at the end of July, 1997, but was killed in the ensuing battle (DH4, 5). His body was never recovered. However, Harry Potter later discovered his magical eye affixed to Dolores Umbridge’s office door at the Ministry and took it (DH13). He buried the eye under a tree in a nameless forest (DH15).
Many advanced skills as a seasoned and very dangerous Auror
"Alastor" = English from Greek, an avenger or nemesis; the implication is that of terrifying but just retribution.
"moody" = describes someone whose outlook or personality changes quite often, usually in a negative way. Alastor Moody certainly fits that description. The word also has an alternate meaning of 'counterfeit' or 'false' [NSOED].
Dr. Lousie Freeman, guest on Episode 11 of the Mugglenet Academia podcast, suggests that Mad-Eye Moody may have been suffereing from (or be a representation of someone suffering from) Post-Traumatic Stress Disorder (or PTSD). As noted in the etymology section above, Dr. Freeman mentioned that the name "Alastor" means "avenger" or "one who does not forget." The inability to forget or push negative thoughts about previous traumatic events from one's mind is a very common symptom of PTSD. This symptom likely plauged the ex-auror and combat specialist Mad-Eye, as he is often seen in memory to bring up past events. Further, Dr. Freeman noted that "hyper vigilance" is another common symptom experienced by PTSD sufferers. Moody's (or the imposter Barty Crouch, aka "Little Crouch") personal slogan of "constant vigilance" may relate to this aspect of PTSD. --Hufflepuffskein