A.E. Pessimal was a government inspector assigned by Lord Vetinari to inspect the Watch and judge whether the Watch gave fair value for the civic funds it used (who watches the watchmen?) (Thud!) and to inspect the Unseen University in "A Collegiate Casting-Out of Devilish Devices". He is described as a neat little man, with very shiny shoes, having no friends and no sense of humour. He does not have a first name, as others would understand; he was 'initialled' at birth rather than named. He was in a position to seriously inconvenience Commander Vimes with difficult questions such as "Why is C.W.St.J. 'Nobby' Nobbs in the Watch? Are you aware you employ a petty criminal?"
Eventually, Vimes decided to shake the man up, and swore him in as a Special Constable for the duration of an impending street fight of roughly a thousand trolls and dwarfs. Vimes thought he could scare the man while showing him what it was like to be a copper. Instead, A.E. took his position seriously, to the point of bare-handedly attacking and trying to bite a troll who took a swing at Vimes, being one of the only two Watchmen injured during the fight (the other being Vimes). This action remains one of the few events to have totally shocked Vetinari, when he heard about it in Vimes' report ('Mr Pessimal? Mr A.E Pessimal? We are talking about the same person? Small man, very clean shoes?').
A.E. later came to Vimes' office. Vimes sensed what the inspector wanted and offered the position of Lance-Constable and adjutant, with an estimate that he could be a Sergeant in a year. Vimes' reason for taking A.E. on was A.E.'s patience and intelligence; Vimes needed someone who could look through paperwork and understand what was being said by sifting out important or suspicious facts. Vimes reasoned that A.E. always wanted to be a Watchman and was stopped by his weak stature. As a condition of his employment, A.E. was told that he would go on patrol twice a week, so he would be able to learn what's important. A.E. is also one of the few people Vimes allows to call him "Mr. Vimes," saying that the man "earned it all in one go."