Sasami Jurai first apeared in the series called Tenchi Muyo! Where she portraied an 7-8 yrs old girl that's in love with Tenchi, the main character. She's the younger sister of princess Ayeka, thus 2nd princess of Jurai. She comes to earth to live in the Masaki house with all the other characters. She loves cooking and is very responsible for her age. She has a pet called Ryo-Ohki which is extremly close to her. Later in the story you learn that she died when she was very young. That's when Tsunami combined herself with Sasami to bring her back to life, giving her great powers she doesn't know about. When Sasami grows up she will become Tsumani.
In a different galaxy... Where Sasami is known as Sasami Kawaii(means pretty) Pretty Sammy was born and had her first appearance in the Mihoshi Special
After that she starred in 3 OAV movies. and subsequently got an own TV Series with 26 Episodes. Sasami Kawaii, a 4th grade girl, is recruted by Tsunami to create peace on Earth to help restore balance of Gemini so Tsunami can become queen of Jurihelm. The 2nd in line candidate for queen, Ramia, tries to sabotage Tsunami and Pretty Sammy by sending her little brother Rumiya to transform Sasami's best friend Misao into the Magical Girl Pixy Misa!
In the OAV movies the concept of an "older" Sasami was obviously dificult to realize. Sometimes you will notice that in the same movie Sasami has different sizes. It's certainly not easy to make a character look a bit older. During the opening she even seems to have a teenager body because of her female atributes being drawn excessively.