Tenchi Muyo! Ryo-Ohki:
In the original series, Sasami and her sister Ayeka are the daughters of Emperor Masaki Azusa Jurai and the second of his two wives, second Empress Masaki Misaki Jurai. On her mother's side, Sasami's grandmother is Seto Kamiki Jurai and her grandfather is Utsutsumi Kamiki; on her father's side, Sasami's grandmother is Masaki Amame of Jurai and her grandfather is Yotsuga Kazuki. Sasami has a half-brother called Yosho Masaki Jurai, son of Emperor Azusa and his first wife, Funaho. Shortly into the series, Sasami and Ayeka take to living on Earth with Tenchi's family (Tenchi, his father Nobuyuki, and his mother's father Yosho, now named Katsuhito Masaki), along with the catlike creature Ryo-Ohki and three other women staying in the house. Sasami arrived on Earth by "accident," stowing away on her sisters ship when she went looking for Yosho. She quickly became close friends with Ryo-Ohki and even Ryoko, despite that Ryoko held a blade to Sasami's throat almost as soon as they met.
Sasami holds a dark secret within her past. Seven hundred years ago, during the attack on Jurai by Ryoko, Sasami was playing around in the Royal Arboretum and took a very dangerous fall, and as a result was mortally injured, and she believed that she died that day, that she was now a copy of the "real" Sasami. The truth is that Sasami survived her injuries when Tsunami-kami-sama, in the form of Tsunami-no-ki, assimilated her. Tsunami-no-ki absorbed and re-created Sasami's body in order to heal her... and merged her spirit with Sasami's. In effect, they are now one being with two souls, and as they grow older their spirits will merge. This may account for her personality in this series. This also grants Sasami several powers that she does not possess in any other series. She now commands the most powerful ship in existence—Tsunami-fune—and can summon it from sub-space at will. Through her link to Tsunami-kami-sama she is linked to every Juraian Ship and their owners, and can sense their location and status. She also has precognitive abilities, but unfortunately these manifest as horrible, disturbing and sometimes even bloody nightmares.
Sasami hid her secret from those around her—including herself—because her greatest fear was that she would be rejected by those she loved. When it was finally revealed, though, Ayeka said she didn't care about it and still loved Sasami just the way she was; Tenchi, Yosho and the other girls also accepted the fact.
It should also be noted that, if needed, Sasami is an accomplished combatant, most likely taught by her mother, who is the Supreme Commander of the Royal Bodyguard, and enhanced by the link with Tsunami-no-ki, easily defeating Mashisu Makibi, who was a trained Galaxy Police officer. One of the cherry-like hair bands she wears is capable of becoming a battle staff.
Galaxy Space Mihoshi Special:
In this one-off episode (written as a "pilot" for both Tenchi Universe and Pretty Sammy) Sasami appears as herself, possibly her OAV version, but in the story told her name and image is used for the role of a Galaxy Police cadet, and has a secret identity as Pretty Sammy, along with her pet Ryo-Ohki. She is also known as the best tea maker in the GXP.
In the first TV series Sasami is still Ayeka's sister and princess of Jurai, but there is no mention of her parents, and Yosho (along with his grandson Tenchi) appears to be only a distant relative. Tsunami does not appear in this series, so Sasami has no alter-ego (other than Pretty Sammy). Sasami is still an accomplished cook. She has a much more happy, innocent outlook on life and does not use any special powers. She does—or did—own her own ship, but it was soon destroyed, stranding her on Earth. She is an expert games player, especially fighting games, and takes the revived Azaka as her pupil. During the three part special episodes "Time and Space Adventures," which seem to be homages to various genres or specific shows/films, Sasami is usually side-lined away from the main story, except of course in her own in which she once again plays the role of "Pretty Sammy." It is worth noting that even in her own story she starts off as a background character, only taking center stage once she has transformed.
Tenchi Muyo! in Love 1 & 2:
These two movies are set in the Universe continuity and Sasami's personality remains the same. A manga companion to the second movie, Tenchi Muyo! in Love 2: Eternal Memory, shows what was supposedly happening to Sasami "off-screen" and introduces Tsunami-kami-sama to the Universe continuity, but it is unknown if this is an official account to be considered as taking place in the same continuity as the movie. The manga implies that Sasami & Tenchi will one day have a child named Izumi, but Sasami will die soon after her birth. However, according to Sasami's narration, that was only one possible future. With every choice they make they can change their fate. It's also told that Tenchi might not choose Sasami but she'll still care for Tenchi the same as always.
Tenchi in Tokyo:
As with most of the characters, Sasami's personality became much louder than before. Her moods are more erratic, given to bouts of juvenile anger and sulking. She becomes friends with Yugi, before learning Yugi's true nature, but remains her friend until the end of the series. Ryo-Ohki is also her pet in this series.