Major Hippocrates Brunel, Lord Moonbark, late of Her Majesty's Bonestream Guard (as he introduces himself to Agatha upon his first appearance) is a British spy who specializes in the notes and artifacts of dead Sparks. He meets Agatha for the first time as they are both disembarking in Londinium after traveling from Paris aboard HMA Tom Wilksenson. He is a member of the Queen's Society and 'quite mad', presumably a Spark himself. He is also the of the Royal Collection of Inconvenient Oddities, where he later meets Gil and Trelawney Thorpe.
He states that he has been featured in some of the novels covering Ms. Thorpe's exploits, but immediately goes on to claim that these depictions are highly fictionalized and sensationalist; his fellow British agent Ardsley Wooster reacts rather testily to this comment, implying he has experienced the 'sensational' aspects quite often himself. Wooster may also be reacting to his presence; what history they share has not been revealed, but Wooster against taking him at face value.
There is a Queen's Mirror in the Collection he curates, which he tells Gil and Trelawney was retrieved in pieces from an abandoned temple in the Alps and personally reassembled by himself. He is involved in the conspiracy of Lady Ariadne Steelgarter and "Clank-Lu"; he plots with Lady Steelgarter to help "distract" Agatha and Co. during their visit to England. (Given this, it should be noted that anything he says to any of Our Heroes should be taken with a least a grain of salt.)
Later, he is a cheerfully impressed witness when Steelgarter, Francisia Monahan and "Clank-Lu" are able to connect to the aforementioned Collection Mirror from the similar portal located on Big Rat Island. Before Steelgarter can stop him, he touches the active connection and learns first hand why actually using the passage is for the moment a bad idea; he survives this bout of Science! with cheer unabated. When Agatha and Co. take over the Big Rat side of the Mirror(s), Brunel manages to avoid being seen, but his location is correctly identified by Princess Neena.
Possible Relevant Outside Information
It seems likely that Hippocrates Brunel is named after the real-life British engineer called "one of the 19th century engineering giants", Isambard Kingdom Brunel.
The name of Brunel's old outfit, the Bonestream Guard, is probably modeled on the real-life Coldstream Guards, although, if this is the case, it should be referred to as the Bonestream Guards (since the Coldstream Guards is always referred to in the plural).