Sasami is the younger sister of Aeka. She is also the second princess of the planet Jurai. Though she was born with powers she lost them during an accident 700 years ago when she fell down the room that stored all of the tree's of Jurai. When she fell her body landed near Tsunami the first generation tree of Jurai. Tsunami(also one of the 3 Godesses in the Tenchi Muyo! universe), assimilated herself with Sasami's dying body in order to save her.

From that day forward Sasami and Tsunami are permanantly linked together. They are separate for the time being but as Sasami matures they will join together to because a single person. But for now(as in storyline now) Sasami and Tsunami are separate and Sasami feels that she is not the real Sasami. She kept trying to tell her sister Aeka this fact for 500 years but she could not, until that day when Tenchi and gang went to repair the Inn they destroyed
(OVA Ep.9). Then she managed to tell Aeka. Later that night Tsunami appears and tells Aeka that Sasami is her real sister.

In the TV series(both) Sasami is no longer linked with Tsunami, but is on her own. She's also really close to Ryo-Ohki, they became good friends almost instantly(side note: In Shin Tenchi Muyo! Sasami owns Ryo-Ohki).

The two triangles on Sasami's head are a symbol of status on the planet Jurai. Those that have these markings are considered to be very powerful. Some try to fake these markings using cosmetics but the one's that Sasami has are real, and so are Tenchi's. Sasami is also Pretty Sammy. A spin-off character in the Mihoshi special and in one of the alternate universe episodes in Tenchi Muyo! TV.