Tsunami:

Tsunami is the human form of the Jurai tree and ship.  She is the image of Sasami in the future.  When Sasami was very young, she had an accident which would have killed her.  But because she landed at the base of tsunami (in tree form).  Tsunami took pity on the young girl, and so to save her life she chose Sasami to assimilate with her.  Because of this, Tsunami became Sasami's ship.  Tsunami it the tree from which all other Jurai ships and trees get their power and because she is able to generate the light Hawk wings, she is one of Jurai's most powerful weapons and beings.  When Sasami grows older, they will become one person, with both the memories and feelings of Tsunami and Sasami.  Its is also Tsunami who convinces Tenchi to believe in himself and to believe that he has the power to defeat Kagato and rescue Ayeka and Ryoko, which he does.

Tsunami is one of the three Goddesses from a parallel dimension, the others being Washu and Lady Tokimi.  This is why she is so powerful.  She left her own dimension of her own free will for reasons unknown, she was not forced to leave like Washu.