Yakage
Yakage is a manga only character that has only appeared in No Need For Tenchi! and The All-New Tenchi Muyo! manga series.
Not much is known about Yakage, but it seems that he is from Jurai. Like Ayeka, he had two wooden guardians by the names Kuze & Hakkou. They are a different design than Ayeka's and communicate through beeps rather than words. He has a sentient ship that shares properties of a Juraian Treeship and crystal properties like Ryo-Ohki. He also seems to hold a certain amount of esteem for Princess Ayeka, or her title. Finally, Washu, while noting that she knows that people for Jurai have long lives, wonders if he is as young as he seems.
At one time, he was an assistant in Washu's lab. At this time, He copied Ryoko's DNA and created Minagi using it and his own. Essentially making her a half-sister to Ryoko. He was apparently present when Ryoko was created but she does not remember him.
Yakage masked
His goal in life was to create the "Ultimate Sword" and trained Minagi in the arts. Some time after the incident with Kagato, he hears of Tenchi's use of the Tenchi-ken and wishes to face him in an effort to test his own swordsmanship and learn the secrets of the light-hawk sword.
Whether on her own, or on Yakage's orders, Minagi reaches Earth first and tries to engage Tenchi. However, Ryoko defeats her and leaves her with amnesia. She forms a bond with Sasami and flees the house, feeling she'll hurt her if she stays. Yakage captures her but she is unable to remember him.
Kuze and Hakkou
He and his two guardians attack the Masaki house in an effort to battle Tenchi, but he finds that Tenchi to be a too inferior opponent. Ayeka also attecks him but she is easily defeated and captured. He tells Tenchi that, if he wants Ayeka back that he should seek him out and face him once he has trained more. The conflict with him forces Tenchi to learn how to become more one with the Tenchi-Ken and his powers.
After a failed rescue attempt by Ryoko and a period of intense training, Tenchi joins Washu, Ryoko, and Mihoshi in a second rescue attempt. First, Minagi fights Yakage, saying a fight with him would be the only thing to bring her memories back. And while he defeats her, she does remember her time with him and rejoices that her master was able to defeat her with what is obviously a replica light-hawk sword.
It is eventually revealed that he was the last of a long line of clones and desperate to finish his work, wishing to pass on the secrets of the ultimate sword to Minagi. It seems to everyone there that he has succeeded in his goal with his replica sword but he insists that something is missing and the only way to find what it is, is to fight Tenchi. He sets Ayeka's cage to shrink and kill her (Ryoko as well, who attacked him again and was captured as well) unless Tenchi kills him.
Yakage is able to defeat Tenchi's basic Tenchi-ken and it seems he will kill him, but Ryoko breaks out and covers him. When Both he and her are threatened, Tenchi revives and brings forth the true Light-Hawk Sword. Tenchi defeats him and as he realizes he has lost and at the end of his life, he decides he can find solace in being defeated in such a fashion, proving that no replica can defeat the true sword, justifying his life as a warrior and scientist.
After the fight, they also discover that Yakage did not kill Minagi, only leave her and her wounds in a state of suspension. after finding a place to bury her master and father, she decides to become a "noble" space pirate, only attacking evil people.