Washu OVA First of all little washu is actually a grown woman who has reverted a physcial form to that of a twelve year old girl. Many years ago Washu came to the "Tenchi Dimension" from another dimension, in which she was one of three goddess. She has two sisters, Tokima and Tsumani. After she arrived she fell in love and married, she even had a child, but since she was in a different call than he there marriage was disolved and her husband and child were taken away from her. She couldn't understand why this was done. She then came to a conclusion: that if that is how the adult world works then she would not live in the adult world... she would remain a child.
Little Washu is also a super genuis. She knows this very well and has no problems about boasting that she is the greatest scientific genius in the universe. In the OVA she was a famed Professor at some well known academy until she was kidnapped and held prisoner by one of her students, Kagato. Kagato stole Professor Washu's greatest inventions for use in an evil scheme to rule the universe. He brainwashed Washu's child, Ryoko, that she created using her knowledge of genetic engineering, and the cabbit Ryo-Ohki. Both where sent on a rampage across the galaxy destroying all in there site until they reached the Planet of Juria, sometime after they where deveated and imprisoned on Earth. Washu remained in a state of supensed animation until being released by Mihoshi.
After being released she took up residence in the Masaki home and showing a very obvious interest in Tenchi
Washu in the "Tenchi Universe" In the first Tenchi TV series, called the Tenchi Universe Collection, Washu takes the place of Ryoko, being imprisoned in a meteorite that is guarded the Masaki Shrine. Why she was placed there is fairly obvious, a great counsel feared her power and called her mad scientist. Thus she was banished into space. This time she was released by Ryoko of all people during a battle between Ryoko and Ayeka over what else... Tenchi.
After being released in a dramatic fashion she chased everyone out of the cave. Which was easy since they believed she was a goblin. She would have chased Tenchi (who was tied up at the moment ^_^) and the gang all night if Grandpa Masaki hadn't recognized her and insisted that she stop. After conversing with him briefly she turned Ryoko into a water sprite for knocking her around. And then she turn Ayeka into a water sprite as well for calling her silly. "What are you talking about? Ain't no goblin cute as me!!!," cried Little Washu before taking up residence in the Masaki home.
Washu in "Shin-Tenchi-Muyo" or "Tenchi in Tokyo" In this version of the tale Washu and Ryoko are space pirates running from Mihoshi and Ayeka who crash on Earth in an attempt to find some great source of power that would help them evade detection. Sorry that's all I know... I haven't seen "Tenchi in Tokyo"