Harlequin was an apparently skilled British spy and magician who had been stationed in the Czech capital of Prague. His real name is unknown, perhaps even to himself. He was shot and killed by Czech infantry when they attempted to arrest Nathaniel.
The majority of Harlequin's history is unknown, all that is known is that he was stationed in Prague to keep the British Government informed of magical activity, and possibly about the Czech government's activities.
During the events of The Golem's Eye, John Mandrake (Nathaniel) was sent to Prague by his master, Jessica Whitwell, in order to find information on the Golem that was terrorizing London. The second Secretary of the Foreign Office told him that his contact in Prague would be Harlequin, and described him as being, contrary to his codename, an old man with morbid tendencies. The secretary informed Mandrake that he must wear a blood red feather in his hat, and meet Harlequin at the Prague city graveyard at night, where he would recognise Harlequin by his "distinctive candle".
Harlequin arrived late for the meeting, much to the chagrin of John Mandrake. He arrived carrying said "distinctive candle" (implied to be held by a severed hand), and put on a theatrically sinister show of greeting the young magician, though on numerous occasions during the encounter accidentally dropping this act. He made the meeting difficult by being pedantic about the colour of Mandrake's feather, and had a small argument with him about the necessity of using the "distinctive candle" for magical purposes, Mandrake claiming it fancifully Gothic, and unhygienic. Using the candle, he made a preliminary magical sweep of the graveyard to detect any spirits in the area (causing Bartimaeus to dive into a semi-open grave to evade it), and then listened to Mandrake explain the situation in London, and being genuinely surprised by it.
Expressing doubt in the Czech's ability to create Golems, he promised to give Mandrake contact names when they met again the following day. Just as he was about to tell Mandrake where they were to meet, the boy irritably warned him against suggesting places like "Gibbet Street" or something like that. After indignantly denying he would suggest such a place, he said he was thinking more along the lines of the old plague pits, which Mandrake thought was no better. Dropping his sinister manner entirely, he dismally suggests the town square near the hot dog stand, remarking "That mundane enough for you?" before disappearing dramatically into the dark.
Harlequin made his final appearance as the hot dog vendor, though only Mandrake and the Czech infantry knew it was him. Just as Mandrake received the contact names from him, concealed cleverly in the hot dog he had given the young magician, the Czech authorities ambushed the square, and Harlequin attempted to flee on his bicycle, tethered to the mobile brazier. He was shot and killed by a Czech soldier as he attempted to pedal pass them.
His death was mentioned by Henry Duvall later, when Mandrake was being questioned by the senior magicians. Duvall mentioned him to be one of their best spies.
He was mentioned by the Second Secretary of the Foreign Office as having conformed somewhat to Prague's general mood. He was morbid to a degree, and seemed to enjoy theatrics, as he demonstrated during his and Mandrake's first encounter. Any attempt to dispell his drama was met with indignity and annoyance. He seemed to loathe the normality of Mandrake, noticeably put out when he gave in to Mandrake's insistence that they not meet in any dreary or death related areas.