Beelzebub is one of the fallen angels that has rebelled with Lucifer and is Lucifer's second-in-command. He is very sadistic, even more so than Satan because he personally oversees the torture in Hell.
He has a friendship with Myishi and he likes to collaborate torture with him. However, even Beelzebub is somewhat disturbed by Myishi's sadism.
Beelzebub first appears right in the second chapter when Meg and Belch have both died and Belch winds straight up in Hell. However, Meg is lost in the tunnel, and Beelzebub is disappointed because he was tracking her through Hell but lost her last minute. Beelzebub now wants to find her and damn her forever. Beelzebub uses Myishi's CCTV in the tunnel to follow Hell-bound souls and he is annoyed one of the cameras blanked out and he missed Meg's destination.
Beelzebub is in a meeting with Satan when this happens and he reports to Satan he missed the new soul. Satan is angry and threatens to kill him for being too old and slow for the job now. But Beelzebub knows what happens to old demons (said to be so terrifying even Eoin Colfer doesn't describe it) and he pleads with Satan for a second chance and that he won't flunk it. Satan threatens to Beelzebub he'll fire him if he messes this one up and that he'd move his home into the Dung Pit instead. Beelzebub promises he'll do his best this time. Satan is tired of excuses and he asks Beelzebub to hurry up and get Meg.
After, Beelzebub sneakily rings St Peter, asking for help, and Peter is annoyed Beelzebub can't do anything on his own. However, he gives Beelzebub a file on Meg Finn, and Beelzebub is shocked at Meg's data. However, Beelzebub knows that if Meg goes back to Earth, she could possibly destroy the planet, and so he promises to get her back to the Afterlife, and tells Peter he will. However, he betrays Peter by not telling him he'll send Belch back too to hunt down Meg with a Soul Man, an electronic demon. Belch is far more powerful than Meg so he could easily cause more damage. However, Beelzebub admits to himself he is a demon, and would St Peter expect full collaboration from him?
Later, Beelzebub goes to meet Myishi, and he tortures Belch with him. He finds from Belch's memories that Meg protected Lowire McCall from being shot and this tiny act saved her from Hell. Beelzebub says that the odds of this happening must be a million, but Myishi says 89%, actually, which infuriates Beelzebub.
However, Beelzebub tolerates Myishi for as long as he is useful to him, and when Belch is rapidly thrown back to Hell by too much positive energy Beelzebub orders Myishi to fix Belch's brain again. Myishi says the dog half of Belch is taking over his human half. Myishi resurrects Belch but leaves him as looking like a werewolf. Beelzebub sends Belch back to Earth to Franco's apartment, and tells Myishi that if he flunks this, he will be in Hell's sewage forever.
After Meg beats Belch to the Cliffs of Moher for the final wish, and gets Lowrie there in time, she is given energy crystals by Flit, a ghost tunnel worker, and Meg's final act of kindness (which is reviving Lowrie and Franco with the crystals) earns her entry to Heaven. However Belch tries to damn her to Hell, but she kicks him on the snout, causing him to fall into Hell forever. Meg goes to Heaven, much to Satan's anger.
Beelzebub is unconcerned really, and he does say to Satan he has learned a valuable lesson, not to trust human technology, and he has shut down all Myishi's CCTVs. Beelzebub says that Myishi is getting too arrogant by far, and Satan recommends that Myishi be thrown into the sewage as punishment. Wanting to keep in Satan's good books, Beelzebub hastily agrees and moves off. However, Satan calls him back, and says that a Gothic movie director is due to arrive in Hell later that day and asks Beelzebub to personally welcome him, because Satan thinks the director will polish up Hell's decor. Beelzebub also agrees and Satan threatens him not to flunk this one.
Beelzebub is a demon with somewhat more human feelings than Satan. He is easily fearful, of his master Satan and oblivion, and Beelzebub will even side with Heaven rather than face obliteration. Beelzebub is mentioned to have actually saved humanity three times by siding with Peter - they have averted a world war and two presidential assassinations.
Beelzebub is tolerant of angels such as Peter, because he knows, as does Peter, that if Beelzebub were to be replaced the next number two may not be as accommodating. So Beelzebub is a sycophant with Satan, which often annoys his boss.
Beelzebub has a very warped idea of evil, he says "evil is evil" which annoys Satan. Because Satan has too many impulsive sinners, who commit one major sin and they are dead, Satan wants a creative sinner, who will evade the law for years and spread the "gospel of misery." Beelzebub is treated to this lecture every millennium yet somehow he never learns better.
Unlike most incompetent minions, Beelzebub is still alive at the end of the book because he has avoided being incinerated by Satan.