Honorius was an Afrit of the 9th level. He was considered very powerful, and was immune to the magics of Djinn and lesser spirits.
Honorius was first mentioned in the prologue of The Golem's Eye, during the British conquest of the Holy Roman Empire. During the battle, an imp reports to the Czech authorities that the British army is being lead by Gladstone's personal afrits, Honorius and Patterknife. During this time it can be assumed that Honorius fought against the Czech controlled Golems, as he later in the book displays distinct rage at the presence of one in London.
Chronologically it is implied, and later pointed out that Gladstone bound Honorius into his body with his last breath and charged him to guard his tomb in Westminster Abbey for all time. In mere weeks of Gladstone's entombment, several magicians, including the erstwhile Prime Minister's private secretary, descended into the tomb to attempt to steal Gladstone's objects of power. Honorius, being charged specifically to guard Gladstone's items, body, and then to kill any intruder (the plaque on the tomb itself explicitly mentions, in ancient Sumerian, that there was a terrible guardian, a horrid death to any intruder, and that no apologies were accepted), slew them all, lined their bodies against a far wall and placed a glamour in the form of a wall in front of them to hide them from view.
During the present time of the series, he was awoken by members of the Resistance, who were looting the tomb at the time. Stanly was slain first and fell into the tomb itself. After revealing himself to the rest of the group, he attempted several magical attacks only to find each had magical resistance. One by one he killed each member, presumably with the Sword of State which was also held in Gladstone's tomb. Only Kitty and Nick Drew survived, Nick having run at the first sight of the skeleton, and Kitty having managed to evade him until Honorius cornered her in the cloister of the abbey. Kitty was able to fend the demon off with her silver pendant but was ultimately saved when Honorius was distracted by the night sky. Lost in thought, Honorius apparently forgot her, took all of the Gladstone's possessions except the Staff (which Kitty still had) for safe keeping and sprang off into the city of London, causing general mayhem.
During the two hours it took the government to gather sufficient forces to track and destroy Honorius, he had rampaged across the city, killing many commoners who he thought might have been Kitty and Drew, telling any spirit he met about the joys of the lack of pain inside the skeleton he was inhabiting, and generally causing mayhem and panic. The djinni Bartimaeus located a group of Imps who had encountered the afrit as it danced whimsically passed them, one of them being eaten each time. When Honorius returned, Bartimaeus took the form of an imp and subtly questioned Honorius about the break in to Westminster abbey while at the same time sending discreet gestures to two Djinn skulking on a nearby roof. Eventually tiring of talking, Honorius attempted to eat Bartimaeus. The djinn nimbly evaded the lunge and attempted to send the afrit careening over the edge of the building. At the same time the two djinn on the other roof acted and all three sent spells at the afrit, only to be rebuffed.
Honorius sped off across London again, though he was shortly caught again by the three djinn, who came at him with railings that held silver items on their ends. Retreating steadily, he eventually tried to escape by diving into the river Thames, where he seemingly perished.
Honorius later reappeared when Kitty Jones led John Mandrake to the Staff of Gladstone. Honorius was found near Mandrake's car, apparently having killed the driver. He decided to kill all present, including Jacob Hyrnek, apparently on a whim but was distracted by the appearance of the London's Golem, who was there on the orders of Duvall to seize the staff. Honorius attacked it viciously, ranting at it in Czech things like "I defy you!". Eventually Honorius leapt upon it and clung to the clay man. This caused the afrit to violently explode the earth magic animating the golem reacting violently with the fire and air aspect of Honorius' essence.
Honorius' original personality is unknown as he is only seen during the present at which time he is insane. He has a very bipolar sort of nature, being extremely whimsical and carefree then suddenly turning murderous for no apparent reason and vice versa. Most of the time he is easily distracted or goes into ramblings about arbitrary thoughts and opinions and complaining about his long deceased master.
He seemingly has no real interest in following his charge as he was quite happy to forget Kitty when he had her cornered in the crypt, instead bursting out into London and basking in his freedom. He still carries knowledge and opinions, though the manner in which he communicates them make those around him believe them to be insane ranting. The majority of his acts are the work of a madman; literally dancing across rooftops, making amusing noises with the bones he was inhabiting, and shouting "Give me back my face!" when Bartimaeus stole the death mask he was wearing. Possibly his sanest act, as put by Bartimaeus, was when he deliberately leapt onto the Golem and perished, apparently tiring of his sorry existence.