One of the Choushin and the Assimilated Partner of Sasami.
Tsunami is one of the Choushin, one of three nigh-omnipotent goddesses who created the very multiverse. It was her influence that allowed Jurai's influence in the universe to flourish as she created the Royal Trees used to power Juraian spaceships. Tsunami hoped to find the power of a higher being by having it reveal its power through someone born of the Juraian bloodline. Over 700 years ago, Tsunami saved the life of Sasami, who was dying from Ryoko's attack on Jurai, by assimilating with her thus giving Tsunami a human form. Tsunami was summoned by Sasami to combat Kagato and later explained her assimilation with Sasami to Ayeka and Tenchi. She was also summoned to prevent Z from killing Tenchi and sustaining control of Tenchi's power before it destroyed the universe.
In spaceship form, Tsunami can form 10 Lighthawk Wings as a powerful defense and has powerful offensive capabilities. The ship protected Earth from a planet-destroying blast fired by the Soja and crippled the ship's defensive systems. In her true form, Tsunami is capable of existing in multiple dimensions and her very presence threatens the stability of the universe if she were to engage in combat.
Tsunami makes an appearance in the manga adaptation of Tenchi Forever called Eternal Memory. Sasami enters into her chamber while being persuaded by a girl named Izumi. Sasami sought Tsunami's power to learn of a means to save Tenchi from Haruna's world. Tsunami has Sasami reflect on her feelings for Tenchi and makes her realize that Tenchi must make the decision himself to be freed from Haruna's world. Despite this, the two do assimilate as they did in the OVAs and Sasami made use of Tsunami's power to allow the girls on Earth passage into Haruna's world to save Tenchi.
Tsunami is one of three successors chosen to be the queen of the magical land known as Juraihelm. Chosen to be the next in line as queen of Juraihelm, Tsunami had to choose a human from Earth to be her representative. She chose Sasami Kawai as her representative and gave her the power to transform into the magical girl, Pretty Sammy. Unlike her courteous and eloquent demeanor in the OVAs, Tsunami is more carefree and a bit ditzy in her behavior for both the OVA and TV series to Pretty Sammy as she is oblivious to Ramia's plans to sabotage Tsunami's chances of being queen and can be fooled rather easily by the comical villain.