Bilious is the Oh God of Hangovers who makes his appearance in Hogfather. His believers, if they can be called as such, are the people who wake up suffering a hangover after a night's drinking and moan "oh god...". Bilious was created from the excess belief that spills over when Teatime tries to murder the Hogfather. Bilious suffers from a severe and continuous hangover, with all the associated symptoms; headache, nausea, impaired vision, fainting, etc. He dresses like a young man returning from a classic Animal House toga party or a counterpoint to the classical Greek image of Bacchus, with the addendum that his toga is colourfully decorated with the contents of his stomach. Although hangovers are common everywhere in both Discworld and Roundworld, he is a relatively minor Discworld god. Not surprisingly, since he is the one who gets the hangover, he hates people who drink alcohol and he particularly hatesl the god of wine, Bibulous (paterned after Roundworld's Bacchus) , who is probably not even aware of Bilious's existence. The lucky Bibulous feels the positive effects of drinking wine, while all the negative effects are transferred to Bilious. when Bilious is brought before the wizards at Unseen University by Susan Sto Helit, the wizards (correctly) reason that the reverse must also hod true and that if Bilious drinks a hangover cure, he will feel the positive effects while the negative effects (the hangover) be transferred back to Bibulous and the other drinkers. The Wizards create a potion composed of all the things that are said to have some vague effect on hangovers, including Wow-Wow Sauce. The potions cures Bilious of his hangover and which point he considers going on a real bender to well and truly get revenge on Bibulous; dissuaded when Susan tells him there is no time for that pettiness.
Most of the minor gods created during the Hogfather's absence vanish once the Hogfather returns, however it appears that Bilious remains. He meets Violet Bottler (one of the Tooth Fairy's asssistants) at the Tooth Fairy's castle and they clearly fall in love. Pratchett uses the idea that something will exist if you truly believe (ala Tinkerbell in Peter Pan) many times in his novels so t it is logical to believe that Violet's belief and love for him keeps him going, as may the fact that gods can never truly die, even if they have no believers (Small Gods). They appear to be setting up together by the end of the novel.
Bilious is portrayed in the television adaptation of Hogfather by Rhodri Meilir.