Tennyo Masaki, is a supporting character within the official continuity of the Tenchi Muyo! Ryo-Ohki series.
Tennyo is a near-exact double for her and Tenchi's mother, Kiyone. Despite her youthful looks, Tennyo is 80 years old, and like her grandfather Katsuhito (Prince Yosho), she often uses an older appearance when among Earthlings. Because of her similarity to their mother, Tennyo would often swap places with her and take care of Tenchi when he was younger. She left Earth right after Kiyone died, only to reappear when it came time to introduce Tenchi and his extended family to his fiancee, Noike
Once she had arrived back on Earth, she was supposed to pretend that Tennyo was Kiyone as a part of one of her mother's jokes. However, Tennyo explained that since she is 80 years old, it would have caused an uproar if she had stayed with Tenchi since some of her old classmates were still around. Katsuhito later explained to Tenchi that the members of the Masaki family are forbidden to reveal their true identity to the people of the Earth since Earth's civilization is still in the development stage, which is the main reason why Tennyo had to leave.
After introducing Noike to Tenchi and conversing with her brother and grandfather, Tennyo left to go on one final trip with her old classmates to Hawaii and came back with her grandmother Airi just before her father Nobuyuki was to be married to Rea.
In OVA 4, it is shown she is dating a young man, whom she brings to her father's wedding.
Judging from information from series creator Masaki Kajishima's doujins, Tennyo works at the Galaxy Police for Airi or Lady Seto, the infamous "Devil Princess of Jurai." She also designed the armaments on the original Kamidake (Seina Yamada's ship) seen in the OVA spinoff, Tenchi Muyo! GXP.
In OVA 5, Tennyo is revealed to be trying to straighten out the mess left behind by her mother and figure out what she learned about Rea. Tennyo also appears to have dyed her hair, possibly to differentiate herself from her mother, so as to be less painful for her family members.