The youkai are the equivalent of the novels' demons. The common run of youkai have all gone mad, but we're told their ferociousness and cannibalizing activities aren't necessarily natural to them. In the old days humans had youkai as best friends and even as lovers, forbidden as that may be. The upperclass youkai that Sanzou and the others must face have an aristocratic air to them, and a certain contempt for the vulgar, lowborn, but nonetheless arrogant human beings. Youkai have characteristic pointed ears and skin markings somewhere on their bodies, like tattoos. Some at least are able to disguise themselves as humans by the use of controlling devices (earrings, bracelets etc) that contain and subdue their youkai psychic power.
Gyuumaoh (ox-demon-king)
A great youkai king 500 years ago who refused to live with humans. He was prepared to plunge the world into chaos and devour the human race wholesale, but was defeated by the heavenly champion Nataku, the designated (and impure) 'prince of battles' among the gods, and the only one allowed to shed blood. Nataku imprisoned Gyyumaoh in his castle in India, in some sort of state of suspended animation. The Saiyuuki action begins because a cabal in India is intent on bringing Gyuumaoh back, using both magic and science. The combination of magic and science is as forbidden as the mating of human and youkai, because it destroys the delicate balance that keeps the world going. The resuscitation attempt upset this balance and released waves of negative energy into the world, which sent most adult youkai mad. This led in turn to an outbreak of murder and rapine by the youkai who'd lived peacefully alongside humans for centuries. The Sanzou-tachi are sent west to stop the scheme to revive Gyuumaoh, and naturally meet youkai sent to stop them.
Gyokumen Koushu (jewel-face princess)
Gyuumaoh's concubine, a conniving woman who begins the project to bring him back and have herself declared his queen.
Rasetsunyo
Gyuumaoh's queen, currently in a trance and imprisoned inside a pillar by Gyokumen Koushu, her rival. By all accounts a gentle person.
Kougaiji (crimson newborn child)
Son of Gyuumaoh and Rasetsunyo, imprisoned with his father 500 years ago but since freed- presumably by Gyokumen Koushu and her scientists. A master of weapons and magical arts and a charismatic leader who can win the blind loyalty of youkai, Kougaiji has no real desire to see his father revived and chaos return to the world. Nonetheless he lends his aid unwillingly in order to have his mother released from the spell under which Gyokumen Koushu has placed her. He and his retainers are sent by her to stop Sanzou from getting to the castle. What with his Hamlet-like internal conflicts, he could never put his heart wholly into the fight with Sanzou. In the fight he sequences gets paired up with Gokuu, from whom he learned, in the course of one of those intra-battle conversations that Japanese heroes are wont to carry on, to change his motivation so that he does only what he desires. What he desires is to free his mother, so he takes on Gokuu a third time in earnest, and gets ripped apart by undiademmed Seiten Taisei Son Gokuu. You can't win. Also Minekura's worldview, as seen through Sanzou's eyes, has very little room in it for noble characters. Sanzou is a Darwinist- 'Good or bad, in the end it's the people with the strongest will who survive'-- and Kougaiji's altruism, like Hakkai's remorse, is a character flaw in this scheme of things.
Dokugakuji (single horn child)
Kougaiji's bodyguard, a tall youkai in the habit of calling his master 'Kou' and 'omae' and patting him on the head, which is hardly the way to talk to (or treat) a prince. Turned up at the castle out of the blue six years previously and was taken on by Kougaiji without question. Fights with a large sword that has an eye in its blade, which he seems to pull out of the air. Doku's big brother act may spring from the fact that he's really Sa Jien, Gojou's brother, and the role comes naturally to him. There are dark canon-bolstered suspicions that he took up with Kougaiji to make up somehow for having killed his mother and abandoned his brother. No Saiyuuki character is complete without a large dose of angst in their past, but Dokugakuji is perhaps the least angsty in presentation, always excepting Gokuu, of course. In the fight sequences, gets paired up with Gojou, naturally.
Yaone (eight hundred mouse)
A herbalist or apothecary or whatever you want to call it. Knows her plants and poisons and exploding materials. She was about to be taken as a sacrifice to Hyakugan Maoh when Kougaiji happened to catch sight of her and bought her for his own service. Not sexual- he employs her in her usual capacity. Yaone is the essence of 'bushi' in spirit and language- samurai to the bone, loyal, devoted, and ready to cut her throat open in atonement when she fails to subdue the Sanzou-tachi. In the fights gets paired up with Hakkai, with whom she trades civilities and polite inquiries as to everyone's health. 'Please die!' she says, throwing a bomb at him which he deflects before attacking her, followed by a 'I hope I didn't hit you too hard. Are you hurt anywhere?' They make, one must say, rather a sweet pair. One doubts that Minekura is going to have Hakkai, that one-woman man, take up with a youkai. Still one notices that the fans, when doing yaoi, do it quite literally by the numbers, the numbers being taken from the characters' names or puns thereon. Sanzou is 3, Gojou is 5, Hakkai is 8, and Gokuu is 9. Lots of 53 stories and 39s and both 38 and 83. In this scheme of thing Hakkai and Yaone are a natural, since the first character in both their names is 8.
Lirin (plum bit)
Kougaiji's younger sister, daughter of Gyokumen Koushu and Gyuumaoh. Distinctly kittenish, with something of a mouth on her. Gives the impression of being mid-teens, nubile but not terribly mature. Her mother doesn't like her, but intends to use her somehow in the plan to revive Gyuumaoh. Gets to sit out the fight sequences in the company of Sanzou, occasionally perched on his shoulders, with neither of them being especially delighted at the situation. 'I wanna join in!' she wails, to which he responds 'Then get the hell off my back!'
Kanzeon Bousatsu
One of the five great bodhisattva who govern the world. Looks like a high-painted woman, but is in fact a hermaphrodite. The essence of 'ijiwaru', or (cheerful) malice, generally aimed at Sanzou. When se shows up to save the wounded Sanzou, se's announced as 'the symbol of benevolence (jiai) and compassion', prompting the equally ijiwarui Hakkai to remark 'Looks more like the symbol of self-love (jiai- different kanji) and licentiousness to me.' All of the above, probably. Se's the moving spirit behind Sanzou's journey to the west, obviously with an agenda of hir own that may relate, once again, to what happened up in Heaven five hundred years ago. The temptation is to see Kanzeon as a stand-in for the mangaka, manipulating the guys Down There for the sheer fun of it.
Nataku
The toushin taishi (god of battles prince), also one of the tabooed children, in this case offspring of a celestial and a human being. The toushin taishi is the only person in heaven allowed to shed blood, but the position is an unclean one which the other celestials abhor and shove off onto people whose lineage is somehow impure. Because of what happened in heaven 500 years ago, Nataku is now in a state of catatonia, evidently self-imposed as punishment for something he did.
Konzen Douji, Kenren, Tenpou
The respective names of Sanzou, Gojou and Hakkai up in heaven. Their adventures are chronicled in the Saiyuuki Gaiden, which runs every other month. More information will be forthcoming when the Gaiden tankoubon comes out.