Ryoohki is Ryoko's little sister.
"HUH?" You may be thinking "That little ball of fur?"
Well, it's hard to see at first gance but they are actally half sisters, both their fathers being the Mass.
The Mass is a small ocean faring group of animals that can turn into anything
and use any higher power that happens to be near them as their brain
(kinda like the different lesser parts of a cell work together)
Ryoko's mother was Washu and Ryoohki's mother was different types of metal and rock
(don't ask me where Washu got *thinking* rock^.~).
Washu acts as mother to both of them since she mixed all those elements together.

The cuteyess thing ever has to be when Ryoohki gains the power to
turn into a human after joining with more of the mass.
She first turns into a grown woman but then decides in later eposides
to take on the form of a three year old girl
(Since, I think, it's not only easier to learn to walk but in human years she is proably around that age)
This gallery is dedcated to her little chibi human form^.^

In Tenchi Muyo
Ryoohki's Age : Arould 3 human years old
Ryoohki and Ryoko's ralation : Half sisters, by the Mass

In Tenchi Universe
Ryoohki's Age : Arould 17 human years old
Ryoohki and Ryoko's ralation : None, though Ryoko acts as her close friend and teamate

Got it? Good:)! I just don't want you to get mixed up
All this happened in Tenchi Muyo okay?
ENJOY!^.^v