Rea Masaki is a supporting character in Tenchi Muyo! Ryo-Ohki!, appearing for the first time in OVA 3. She is the current wife of Nobuyuki Masaki, stepmother of Tenchi Masaki and and Tennyo Masaki, as well as mother of Kenshi Masaki.
As revealed in the Tenchi Ban 13.8, and OVA 4, Rea was an artifical human from Geminar; she was one of three preteen-aged artifical human created to be Seikijin pilots to fight against Gaia. The three were having difficulties, and so, it was planned that one of them would be sent to another world, and there have a child that would have the combined talent for being a pilot that the Artificial Humans, and those summoned from other worlds possessed. While they were testing the equipment that would send one of them to the other world, it was accidentally triggered, and Rea was caught-up in it and sent to Earth, where she was almost immediately found by Kiyone, who took her in. This was apparently just after Tenchi's birth, or prior to Kiyone's pregnancy.
Rea explained everything, including her mission to have a child on Earth who would someday have to return to Geminar, and Kiyone promised to help her land a man to father her child. Rea was raised as a ward of the Masaki family and taken-in as a adoptive younger sister by the late Kiyone Masaki, Nobuyuki's late first wife. Rea had also been taught cooking by Kiyone, so even though she does have her favorite recipes, her cooking style is virtually identical to Kiyone's.On the day of Kiyone's funeral rites, she was checking on young Tenchi when she noticed the boy had disappeared. Everyone present searched around for Tenchi but couldn't find him. It was during this search that Rea thought of something. Taking Nobuyuki with her, they found Tenchi asleep at the entrance to the cave where space pirate Ryoko was imprisoned. Tenchi was forbidden to go to the cave by his grandfather Katsuhito, but always went there anyway.
Rea would end up cooking and cleaning house for the Masakis. Sometime after Ryoko was released, she would move in with Nobuyuki at an apartment near the site of the house that Nobuyuki designed during Kiyone's lifetime (that house had been moved near the family shrine, destroyed when Mihoshi crashed into it, and rebuilt at the same site). When the school which Tenchi attended was destroyed by fire (in actuality it was Ryoko's doing), not to mention the Great Seto Bridge's destruction, she and Katsuhito were called to Tokyo to explain what had happened, at the Japanese Imperial Palace no less. At the last minute, Katsuhito made Rea make the report herself. Months later, she revealed this fact to Tenchi when he came over for a visit, but when he asked her why, she said he would be told about it later.
Later, when Tenchi discovered his mother's true personality of being a huge prankster, he became upset because the mother he remembered was nothing like that. Rea tells him that she feels sorry for his mother because the one he remembers is not Kiyone. Tenchi realizes that he had mixed his memories of his mother with memories of Rea, being that she took over the motherly role after Kiyone's death.Rea married Nobuyuki in the final episode of OVA 3, and the preparations for the wedding are shown in OVA 4. It is revealed that Rea's wedding kimono was in fact the same one Kiyone wore when 'she' married Nobuyuki, and they hoped it would eventually be passed to Kiyone's daughter Tennyo. During the preparations, while bathing with Washu, Ryoko, Mihoshi, and Sasami, Rea reveals her true origins as being from Geminar, and how Kiyone found her immediately after arriving on Earth, and tended the injuries she had received. In the spinoff, Tenchi Muyo! GXP, nearly a year later, it is shown that she is pregnant. The final episode of OVA 4 covers the time from Rea's giving birth to Kenshi, to the time when he is a pre-teen.
The 2nd spinoff, Tenchi Muyo! War on Geminar, centers around her son Kenshi, who has been transported to his mother's homeworld of Geminar. There are two unseen voices, shown at the end of the anime, who were involved in sending Kenshi there, one of whom is suggested to be Rea.