This post contains SPOILERS about the identities about past and future Holy Kings and Shamans.
The Holy Kings and Shamans of Dhalashar
The Holy King of Dhalashar is a figure who's been around for at least a thousand years. During that time, he's gone through forty-two incarnations, each unique. Some have been women, some men; some were old when they ruled, some young; they have been Indian, Tibetan, Nepali, Chinese, etc. However, despite differences in both physical form and personality, their core soul has remained the same.
A second soul follows the King in his reincarnations: the Shaman. The King and the Shaman always have a close relationship, though the exact nature of that relationship varies from incarnation to incarnation: we're given examples of Kings and Shamans who have been parent and child, siblings, lovers, best friends, etc. It is the job of the Shaman to regulate the King's sometimes overwhelming powers. They serve the King by providing stability and are the only one who can calm a King who has gone berserk. Needless to say, the Shaman is invaluable.
HOLY KINGS AND SHAMANS OF DHALASHAR Time Period Holy King Shaman 7th Century
Ranga
The ten year old daughter of an Indian Maharaja and the 14th Holy King. Stubborn, outspoken, and very emotional. She sometimes comes off as spoiled, but cares deeply for her subjects.
Shi Jia-Yun
The son of a peasant couple in rural China. Twenty-three years old. The 14th Shaman. Though he's soft-spoken and easily overwhelmed (he's still getting used to the splendors of Dhalashar), he occasionally manages to hold his own against Ranga. 13th Century
???
... I don't own this volume :(
Madeline
A formidable woman who knows her way around both the battlefield and the workings of a household. Approximately twenty-five or twenty-six years old. The 26th Shaman. She's been searching for her Holy King since she was fifteen. 1800's(?)
Atisha
The 38th Holy King. One of the Kings who began to bring Dhalashar out of the realm of myth and into the modern eye. Photographed with a Russian Czar.
???
No canon information given. ~1960's-1980's*
Atisha
A young Nepali woman with both strong will and feminine grace. The 41st Holy King. Good-natured and affectionate, she has always wondered what her life would have been like if she hadn't been the Holy King.
Yahati
A shy eighteen year old from Kazakhstan. His eyes are blind and his body is covered in burns from a failed weapons experiment by the Soviet Union. The 41st Shaman. He worries that his physical limitations hold him back from properly doing his duty. Present Day
Fuuto/Atisha
A pair of very different fifteen year old boys. Together, they are the 42nd Holy King. Fuuto is generally irritable and outspoken, whereas Atisha is generally quiet and acquiescent.
Mayu
A strangely blank girl who's confined to a wheelchair. The 42nd Shaman. Extremely intelligent, if not quite up on the "human emotions" thing. Her calm personality soothes Fuuto's more firey one. Future (~60 years)
Mahatibha
A childlike young man with strong psychic abilities. The 44th Holy King. With a mental age that seems perpetually stuck in childhood, the only thing he considers "real" is his twin, Amitabha. As a result, he is terrifying unaware of the value of the lives of others.
Amitabha
A playful young man with strong psychic abilities. The 44th Shaman. Despite his fondness for teasing others, much of his life has been spent controlling his twin brother's explosive temper. His airy smile hides a sad maturity.
* Volume 11 states that Yahati was born in 196X, and he died when he was eighteen. Atisha died three years later (the year Fuuto was born, obviously). If we assume the series's "present day" to be around the year 2000, that would place her death around 1985, making Yahati's year of birth 1964 and matching the timeline up perfectly. In an earlier volume, the current Atisha is stated to be the first Holy King in forty years. However, this does not contradict the timeline, since the current Atisha would be the first king to be crowned in forty years. After all, he was only crowned during the course of the manga, and the previous Atisha was crowned when she was quite young-- probably in the late 60's. So that would make his coronation the first one in forty years.
The Holy King's Guardians
The manga's title, Genjuu no Seiza, can be translated as "The Cryptid Constellations." The constellations in question each refer to one of the guardian beasts of the Holy King. These creatures, who often have both animal and human form, are the sworn protectors of the Holy King-- they're also generally gods in their own right (see: Garuda, Hanuman, Naga, Yamantaka, etc). In return for their service as gods and protectors of Dhalashar, the Holy King has awarded each a place in the stars above Dhalashar as a constellation.
[SPOILERS] There's more to the contract than that, though. The King's payment for his guardians' loyal service is in his own flesh and blood. When the current incarnation of the King dies, his guardians tear his body to pieces and eat him. [/SPOILERS]
The Holy King has the power to create a new guardian beast with any creature/spirit who accepts his contract.