Sasami Masaki Jurai and her sister Ayeka are the daughters of Emperor Azusa Masaki Jurai and the second of his two wives, Empress Misaki.
Many centuries ago, Sasami stowed away on Ayeka's Royal Treeship, Ryu-Oh when her older sister decided to run away from Jurai to find Ryoko and in an attempt to search for their missing half-brother Yosho and seek revenge on space pirate Ryoko for nearly decimating Jurai 700 years before OVA 1 began. When the princesses arrive on Earth, they get into a battle with the ressurected Ryoko, resulting in all of them crash landing at the home of Tenchi Masaki. Shortly into the series, Sasami and Ayeka take to living on Earth with Tenchi's family (Tenchi, his father Nobuyuki, and his grandfather Katsuhito Masaki, who is later revealed to actually be Yosho), along with the cabbit Ryo-Ohki who is the reincarnation of the ship that almost destroyed her planet, and the other women staying in the house. She quickly becomes nearly inseparable from Ryo-Ohki. Sasami refers to the other women in the house as sisters.
Like the other members of Jurai's Royal Family, Sasami's connection to her Royal Tree has an extended life span. She appears to be a pre-adolescent girl when in fact she is over 700 years old. She was approximately 8 years old upon leaving Jurai and is still physically and mentally 8 due to Ayeka having kept Ryu-Oh in suspended animation during their search for Yosho. Sasami will continue to grow until she reaches a maturity where she can remain for centuries without aging pass her prime.