Ryoko is known as Ryoko Hakubi, a space pirate who was defeated by Yosho nearly 700 years before her release by Tenchi. Before that, she was a slave to Kagato. Her age is at least 5,000 - although she claims to be only 2,000. She was created by Washu, using one of her egg cells as a base, so she is indirectly Washu's daughter. Washu's DNA was combined with that of amorphous creatures called the Masu to create Ryoko. She has three red gems that are the source of her full energy, which also came from Washu.
True to the meaning of her nomenclature, the gem on the inside of Ryoko's left wrist allows her to summon powerful, ferocious 'demon' spirits, which let physical attacks pass right through them, while the gem on the inside of her right wrist allows her to control the summoned spirits, and the last gem, which goes on her throat, gives her full power. She also has the ability to create false gems, one that she keeps hidden in plain sight on her left ear as an earring. This false gem is not as powerful as the originals, but allow her to balance her power and abilities. False gems also last for only a limited time, while the originals have an inexhaustible supply of energy. In OAV 3, it was revealed that the true gems contain the powers and godhead of Washu-kami-sama (who is one of the three Chousin Goddesses). It is debatable just how much of her mother's power she is borrowing and how much she is capable of using on her own, but her relatively young age (she appears about twenty years old) prevents her from drawing upon their full power.
Other powers Ryoko possesses include flight, teleporting, intangibility, creating energy swords and energy blasts (All regularly used), instantly regenerating missing limbs (Episode 1), creating doppelgängers (Episode 7), telekinesis, creating forcefields, matter formation (manipulating matter and merging with it, as with a stone pillar in Episode 6), complex molecular restructuring (repairing Mihoshi's GXP scanner in Episode 4), hair needles she can fire from her head (Episode 11), and telepathy with Washu and Ryo-Ohki. In addition she can access an advanced internal database, as a group-mind shared with Ryo-Ohki (Episode 14) to constantly overview her exact universal coordinates (and presumably other advanced functions/information in addition). Some of these abilities are known to have been inherited from Washu, while others are likely the result of her Masu genes, or intended design. It is possible that she gained shapeshifting abilities from merging with Zero (Episode 12), in combination with her amorphous masu heritage.
As a character, Ryoko is very emotional. She easily gets upset, and is short tempered, especially around Ayeka. But she's deeply in love with Tenchi, though not fully admitting how deeply until Episodes 5 & 6, when she tells Ayeka how she first met Tenchi, and later when she offers up her own life in place of his. While she was trapped in the cave, she was able to create an astral body (sort of like a ghostly consciousness) and venture to the entrance of the cave. She watched Tenchi grow up over the years, and she wished more than anything to be with him. However, the way she introduced herself to him was terribly frightening to Tenchi, so he distanced himself from her for a long time. Also, when Washu is first introduced she tells Ryoko that she knows how she (Ryoko) feels about Tenchi and says that Ryoko wants to do "this and that" with Tenchi.
Ryoko also has a bit of an ambiguous relation with Washu, her mother. When Washu first appears, she critiques her daughter's sex appeal and asks Ryoko to refer to her as "mommy." As the series progresses, she starts to fear Washu, especially when Washu starts doing some of her experiments on an unwilling Ryoko. In fact, Ryoko becomes slightly cautious whenever Washu is looking at her with a mischievous smile and intimidating eyes.
Physically, Ryoko has no taste buds, and has no need to eat; all the energy she gets comes from the gem created by Washu on her wrist. Ryoko eats to be social, but has a taste for sake. Although she can get drunk like normal humans, Ryoko has the ability to nullify the effects of alcohol, which comes in handy if she has to go into battle. She can also instantly nullify any other toxic chemicals within her body. In addition, Ryoko does not need to breathe, and can survive unaided in the vacuum of space.
Ryoko, when kidnapped and electrocuted by Ayeka, seemed to enjoy it. Ayeka called her a masochist, and Ryoko quipped that she was more of a sadist. Judging by the fact that her ship Ryo-Ohki destroyed 28 planets and 69 colonies when in Kagato's service, and by some of her taunts in the fight with Tenchi, she might not have been joking. Ryoko is also sometimes rather childlike, and can be a bit gullible. Since most of her life was spent either working under Kagato, or sealed inside a cave, she does not have much of a range of experiences. She is also extremely sensitive about her age, being brought to tears when Ayeka called her a mummy.
In "Ai Tenchi Muyo" she is now seen constantly wearing an elaborately designed leotard with matching white leg boots, her hair style is much simpler compared to her old hair style in past "Tenchi Muyo" series.