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A.E. Pessimal is one of the Patrician's inspectors sent to investigate the economics and management of the Ankh-Morpork City Watch to judge how the Watch uses their funds (see Thud!). Pessimal does not seem to have a first name or at least does not understand the question when Vimes asks him. This isn't his first such assignment: previously the Patrician has asked him to investigate, and report on, Unseen University's admission procedures and examination system. (So evidently Mustrum Ridcully must also have done something to irritate the Patrician.) This is discussed at length in A Collegiate Casting-Out of Devilish Devices, a short story whose events are alluded to in the opening chapters of Thud!.

Later in Thud! he is sworn in as a Special Constable and bare-handedly attacks Brick, to the surprise of everyone. After that he is offered a position as Lance-Constable which he accepts (with the promise of accelerated promotion to Sergeant) and probably will assume after he has finished his assigment to investigate the Watch.

Vimes' motivation in employing Pessimal is partly practical: he recognises that a man who will attack a boozed-up troll with his teeth is "born to the badge", and he will always find a job for a born copper. More than that, he needs an adjutant, a man who can "hold a pencil without breaking it", who can make sense of paperwork and interpret the mysteries of statistics that he, Vimes, will always overlook; for while Truth is likely to be hidden in the mountain of paperwork that sits on his desk, quietly reproaching him for his deliberate non-awareness of it, Vimes has neither the time nor the inclination to go mining for it.

It is also possible that he is making some sort of obscure point to Vetinari by poaching one of his civilian clerks, quite possibly turning the poacher into a gamekeeper. (Or quite possibly vice-versa).

It is a tragedy that having earned the right to call Vimes "Mr Vimes" by having saved his life, a privilege open only to few, Pessimal is temperamentally unable to do so. However, it looks as if a new Watch department is in formation, perhaps loosely associated to the reformed Cable Street Particulars], composed of Pessimal and the Gooseberry device.