Lord Downey is current Head of the Assassins Guild and a Master titled Assassin. Downey was Guild's second Assassin and right hand to Dr. Cruces, the previous Guild's Head, right hand for a long time until he finally succeeds him. He is amiable-looking man whose specialty is poison, in particular (it is believed) poisoned peppermints. It is mentioned by Samuel Vimes that his Lordship title was created by Patrician, among other Ankh-Morpork Lords.
In his scholar days, Downey was quite unpleasant occurrence, often boasting and showing off, calling everyone a scag and mistreating younger students, notably young Havelock Vetinari (whom he mockingly nicknamed Dog-Botherer). The status of their relationship in unknown but one might assume it's strained.
Downey, as cunning as he is, became second Assassin to Dr. Cruces and inherits the Guild's Head position shortly after his death.
He takes important role during one Hogswatch week, meeting the Auditors (finding them to be quite dull) and accepting their request on inhumation of the Hogfather. After all, they did pay in pure gold. He was played by David Warner in Sky One's movie adaptation of Hogfather.
Lord Downey often mentions rules and style to be of great importance for the Assassin even thought he himself stated that no one became a great Assassin by always obeying the rules (that is, if one is not caught after disobeying them). Like many people with no actual morals, Lord Downey does have standards.