Sasami:

Sasami is Ayeka's little sister and the second crown princess of Jurai.  (She later gets her own series called "Pretty Sasami", but lets not go there shall we?)  I think she likes Tenchi, but not as much as the other girls (because that would just be disgusting).  She joins her sister on her search for her brother Yosho.  She is very playful, and at first seems quite mischievous and playful, then she just becomes more serious and playful.  She develops a strong bond with Ryo-oki, more so than Ryoko in terms of playfulness than psychic, but Ryoko doesn't seem to mind.  She is the stereotypical little sister, but she does all the cooking for the Masaki household and at times seems very mature.  Unlike her sister, she isn't very argumentative, but she can be very persuasive and authorities when she needs to be.

when Sasami was 4 years old, Ryoko (under Kagato's power) attacked Jurai.  Because Sasami was quite young she didn't fully understand, but knowing something was wrong, she went to talk to the Jurai trees.  Sasami is one of the few people who is able to communicate with the sacred trees.  While Sasami was among the trees, a damaged ship crashed into the palace, causing her to loose her balance.  She fell hundreds of feet and landed at the base of the tree Tsunami, from which all other Jurai trees and ships gained their power.  Tsunami took pity on Sasami, and because the trees and Tsunami were quite fond of the little Sasami (because she was able to communicate with them), she assimilated with Sasami to save her life.  Because Tsunami assimilated with her, she lost her sense of identity, and later "confesses" to Ayeka that she is not the "real" Sasami.  

When Sasami grows up, she will merge with Tsunami, in her new form she will retain her own thoughts, feelings and memories, but she will also have the physical appearance, thoughts, memories and powers of Tsunami.