From Discworld & Terry Pratchett Wiki
An unassuming balding man with an abnormally large head and small body topped by a bowler. Originally a clerk, and most probably a Dark Clerk, he was sent by Vetinari to assist Commander Vimes during the latter's appointment as Ankh-Morpork's ambassador to Uberwald. Has the habit of ending his sentences with "mhm-mph" when speaking.
As Vimes suspected and later confirmed, he is also an Assassin in the service of Lord Vetinari, one of the lordship's clerks who have a talent for crunching skulls as well as numbers. Unlike most assassins, Inigo came from a humble background and was selected for a Assassins' Guild scholarship after being noticed as a gifted amateur in the field of remorseless killing. His bowler hat has a razor sewn into the rim and he makes use of other such deadly implements like palm daggers or shoe blades. When the diplomatic convoy was attacked by bandits, he personally dispatched most of the assailants alone, mmh-mhm.
Despite his fighting skills, he met his end at the hands of Wolfgang von Überwald, AKA Wolf, and his corpse was used to frame Vimes for an attempted assassination of the future Low King.