The goddess Tsunami and her sisters Washu and Tokimi, the Chousin, searched for eons for a being superior to them. Tsunami descended to the third dimension to search for this kami (God).
150,000-years ago, the giant, sleeping humanoid form of Tsunami was found on the planet of Jurai by a man who would eventually become its first emperor After waking up, Tsunami detected evidence of potential in this man. Tsunami theorized that his descendants would produce the kami the Chousin were looking for. Thus Tsunami made a pledge to the man of House Masaki to protect him and his descendants and bless them with great power -- her descendants, the Royal Trees. The mighty power of Jurai made way for the planet and its rulers to become a galactic superpower.
Jurai (Juraisei) is ruled by a royal family, the Imperial House of Jurai (Juraiouke), all of whom take "Jurai" as part of their names after bonding with one of the Royal Trees, seeds of Tsunami's power which grow into sentient trees and bond to ships of the Juraian Fleet. Over time, four separate houses or families formed; Amaki, Kamiki, Tatsuki, and Masaki. The current heir to the throne of Jurai is the son of Emperor Azusa and First Empress Funaho, Prince Yosho Masaki Jurai aka Katsuhito Masaki.
Other notable members of the Jurai family are Second Empress Misaki's parents -- Utsutsumi Kamiki Jurai, head of House Kamiki and his wife, Sero Kamiki Jurai, the infamous "Devil Princess of Jurai," an influential figure in galactic politics as well as matchmaker.
Seto is also the reason House Masaki (headed by Funaho) are current rulers as she stopped the house from becoming extinct when numerous members kept running away. House Amaki is currently a bitter rival of House Masaki. The intentions of House Tatsuki are still unknown.
The planet Jurai is home to a species of intelligent trees, Royal Trees of the Imperial House (Oukenoki), descended from the goddess Tsunami. In fact, the name "Jurai" is a combination of the kanji 樹 tree and 雷 thunder. These trees are capable of bestowing great powers and long life to those Juraian royals who are partnered with them, and are activated by special devices known as keys. These trees are also used to provide power to Juraian spaceships. All power supplied to the tree ships comes from Tsunami-no-ki, Tsunami's tree form. from a subspace network.
Inheritance to the throne is determined by connection to a Royal Tree; the higher the Tree is in status, the higher someone in the succession line. This is why Yosho is first-in-line, due to his connection to 1st-Generation Tree, Funaho. Tenchi's powers as a bearer of the Light Hawk Wings, which are usually generated by Royal Treeships, allow him to be second, as well as his status as Yosho's successor. This is why Seina is 3rd-in-line, as he became connected to a seed of a 1st Generation Tree. Assumingly, Sasami could surpass them all because she's connected to the original Tsunami-no-Ki, but she has not claimed inheritance on these grounds as of yet.
Juraians are also feared throughout the Galaxy for their superhuman strength, plus the ability of Juraians of noble heritage to summon forcefields and discharge energy blasts, even a trained Galaxy Police officer was not considered a threat to even regular civilians.
Juraians are also known for their extremely long life spans, as Ayeka appears to be about the same age as Tenchi Masaki (her grandnephew), even though she is over 714 years older than him. Juraians are granted their extended life spans by their connections to the Royal Trees. "Normal" Juraians have the same lifespan as normal humans. Though Nobuyuki Masaki and others who have Juraian blood but are not, (as far as we know), bonded to a Royal Tree, have expanded life spans, though this may be attributed to their training and body enhancement while at the Galaxy Academy or his ancestry stemming from Prince Yosho.