Tenchi Masaki (柾木 天地, Masaki Tenchi) is the main protagonist of the Tenchi Muyo! metaseries. He is a normal boy until a number of alien girls arrive at his house. Some or all of the girls, depending on the continuity, fall in love with him. He, however, accepts none of their advances. As a gentleman and because he cares much more about the family unit than he does about romantic conquests.
Tenchi Masaki is an ordinary boy living in Japan with his widowed father, Nobuyuki. He is a compassionate, humble, noble, level-headed, respectful young man training with his grandfather as a Shinto priest and as a swordsman. A number of alien girls arrive at his house throughout the course of the series, with all of them expressing romantic interest in the young (but innately powerful) boy. Throughout the series, Tenchi learns hidden truths about himself, his role in the universe, and just how powerful and significant he actually is.
Tenchi initially lives in the city with his father to attend school with his friends and visits his grandfather's shrine in rural Okayama every summer. There he trains to become a Shinto priest and learns his family's special style of swordsmanship. Unbeknownst to Tenchi happens to be particular to members of the Royal Family of the planet Jurai. Tenchi's grandfather Katsuhito Masaki, always told him a legend about a warrior who defeated a demon and trapped her within the family shrine. Tenchi goes into the shrine and awakens the "demon," the space pirate Ryoko, who in a fit of rage enhanced by 700 years of being locked up, attacks Tenchi. After ending her attack on Tenchi, Ryoko attempts to seduce him in order to receive her gems that are attached to the sword that he found in the shrine. Shortly after, Juraian Princesses Ayeka and Sasami arrive on Earth to capture Ryoko. They also plan on finding their missing brother Yosho, who was last known to have chased Ryoko off of Jurai (making him the "warrior" of Katsuhito's legend).
During the ordeal, Princess Ayeka is shocked to discover that Tenchi is in possession of the Master Key, Yosho's sword also called "Tenchi." After Ayeka notices that Tenchi is able to use its power, she determines that they are related by noble blood. Tenchi also makes friends with Ayeka's little sister Sasami, and they begin their older brother/little sister relationship.
Tenchi's house is permanently relocated next to the shrine after the battle between Ryoko and Ayeka. Both girls fall in love with Tenchi during their stay at his home. Additionally, Tenchi no longer attends school after the high school was decimated during the encounter with Ryoko in OVA 1. Feeling responsible for his old high school being destroyed and theorizing that trouble will just follow him again if he were to return. He doesn't see his high school friends Kamikura, Ikeda, and Kazuhito that much.
Subsequently, Tenchi suffers a great deal of stress as he tries his hardest to be a gentleman. This is because most of the women within the household take any opportunity to display their attraction for him. Though Tenchi refuses to get intimate with them, he treats them all like family, willing to risk his life for each and every one of them.
More serious trouble comes for Tenchi, when ruthless space pirate and "Ruins Buster" Kagato arrive in order to take the claim of Ryoko. Tenchi, after learning his grandfather's true identity as the Juraian prince Yosho, puts his life on the line to rescue her. During the climax of the battle when Tenchi is thought to be dead, both Ayeka and Ryoko reveal their true love for Tenchi. The both of them going after Kagato in a suicidal battle to the death. After his life is saved by the goddess Tsunami, it is revealed that Tenchi can create three Light Hawk Wings, the most potent sources of energy in the universe, which are usually only created by the mighty Juraian Royal Tree ships. In a final duel, Tenchi defeats Kagato.
After the battle with Kagato, Tenchi welcomes new a houseguest, the universe's number 1 genius Washu Hakubi. She not only takes a huge interest in Tenchi because of his innate power, but also desires for him and only him to give her a child.
At the end of the OVA. Tenchi once again produces his Light Hawk Wings to save Ayeka, Mihoshi, and himself from a black hole during a confrontation with Dr. Clay.
Nearly a year later, Tenchi is shocked to discover that he has an older sister, Tennyo Masaki. 80-year-old Tennyo arrives to introduce Tenchi's official fiancee, Noike Kamiki Jurai. The marriage was arranged by Jurai's political advisor, Seto Kamiki Jurai. In reality, Seto commissioned Noike to monitor the Masaki house as it has the highest concentration of power in the universe, thanks to all its inhabitants. Tenchi is also introduced to his grandmother, Airi Masaki.
Amidst all the chaos, Tenchi learns from his grandfather Katsuhito, that since Earth is a developing civilization. The Masaki clan must keep their true identities hidden since they do not age at the same rate as Earthlings. And those who are born into the Masaki family on Earth are kept from the secrets until they come of age.
During all of this news and confusion. Mihoshi's jealous brother Misao concocts a master plan to kill Tenchi whom he thinks is hosting a harem, with his sister being his sex slave. After Misao's plans are thwarted, Tenchi's life is once again threatened as Z, a man who, like Tenchi, can generate Light Hawk Wings, attacks him. During the battle with Z, truths are revealed including more information about Tenchi's true nature. Tenchi is revealed to be a "potential," a person who can wield power from a higher dimension, ie: the Light Hawk Wings. For eternity, three godly sisters: Washu, Tsunami, and Tokimi -- the Chousin -- have searched for a being superior to them, and Tenchi is revealed to be that being after he defeats Z. Tenchi's true power is seen when the universe is almost destroyed by the impact of his power, if not contained by the Chousin. Tenchi's godly self hatches from his shell and is able to calm Tenchi down, reminding him of the girls he fights for.
After the entire ordeal, the truth behind everything that has happened was linked to the three. They decided to change the timeline in order to repair the damage that they have done and like before, Tenchi gets knocked into the past. This time, however, it didn't stem from the attack on him made by Z, but by the canon of the powerful ship known as the Chobimaru. The reason for this is because if Tenchi didn't go to the past, he wouldn't have saved a little girl who would eventually become Noike. Additionally, in this new timeline, Tenchi creates six Light Hawk Wings, instead of his usual three that he limited himself to.
The OVA begins with the preparations for Nobyuki's wedding, as seen at the end of OVA 3. Tenchi is introduced to his aunt Minaho for the first time since his mother's funeral, as well as several of his cousins who have been in space for the last while. The cousins visit Tsukiko Masaki, Kiriko and Kai's mother, and suggest she has a romantic interest in Seina, who has recently gone to space, indicating this is concurrent with the early episodes of Tenchi GXP. Minaho and Tenchi take some time to get to know each other, with Minaho joking that she was thinking of making Tenchi hers (her husband).
Although GXP takes place in the official continuity. (Taking place one year after the 3rd OVA series and during the first three episodes of OVA 4). Tenchi doesn't play a major role in this series though his unlucky childhood friend Seina Yamada is the main protagonist. The Galaxy Police did try to recruit him, but they ended up with Seina instead. Tenchi appears in Episode 17, where Seina comes to his house to see Washu to talk about Fuku, the cabbit that she had given him. Tenchi and Seina discuss their similar troubles with a harem of girls all fighting for them and also show off their superhuman feats with a race. Tenchi showed that he can move incredibly fast during a race with Seina. He can keep up with him despite Seina having drastic Galaxy Police body enhancements and going downhill on a bicycle. It is somewhat clear that at this point that Tenchi has much more confidence in his mysterious powers as well as his role as the "head of the house" and he's not interested in joining the Galaxy Police. Tenchi, as well as the girls, also appear in a brief cameo in Episode 26 where they attend Seina's wedding.
In the GXP Light Novels, Tenchi and Seina have a heart to heart talk, where Tenchi reveals he has rendered all of the people closest to him effectively immortal.
Tenchi does not appear in this series but his younger half-brother Kenshi talks about him and his "sisters" at his older half-brother's house. In Episode 7 Kenshi mentions that he has several sisters-in-laws. Meaning his older brother Tenchi has at the very least two or more wives, as Kenchi only has one older brother and one older sister by blood. Background info indicates Tenchi (and his family) are watching everything that happens, with the help of Tokimi.
Tenchi makes these alien girls part of his family. As a result, Tenchi isn't lonely anymore, despite the troubles the girls may cause. While Tenchi is a normal teenage guy in that he finds the girls attractive, his desire for a family is greater because they helped him deal with the great pain in his heart because of losing his mother.
So no matter who one's favorite girl is, Tenchi is currently going with the "I'm not choosing a single girl since I like having all the girls around me as family strategy. One of the reasons for Tenchi not choosing a single girl is that if he did, chances are the others wouldn't stay, drastically altering the series dynamic.
Though Tenchi is not bonded to a Royal Tree, he is considered a contender for the throne of Jurai (determined by-election of the four royal families) due both to his status as the successor to First Prince Yosho (who was a contender due to his connection to his 1st Generation Royal Tree) and his singular ability among the Jurai to create Light Hawk Wings on his own. Tenchi is second in line behind his grandfather, followed by Seina who was bonded to a 1st generation tree seed.
It has been confirmed that Tenchi will be around for hundreds of thousands of years thanks to his great power and that Tenchi and the girls will eventually move to Jurai. In fact, in the GXP light novels, Tenchi reveals to Seina that since his ascension, he and the girls have become immortal. Beforehand, however, while still on Earth, Tenchi's relationship with the girls will become intimate (revealed in Masaki Kajishima's doujinshi 'Omatsuri Zenjitsu no Yoru - Omake)
Kajishima has stated that should GXP and OVA 3 do well, that he would consider another series to explore Tenchi's love life. It also should be noted that ALL of Kajishima's works end with a harem in which the guy marries (or otherwise ends up with) many if not all of the girls from that series.
It has been revealed through Kajishima's doujins and pictures that Tenchi will have, at the very least, two sons, born in the same year; a son, who is seen talking with Masaki, sister of the first emperor of Jurai, sometime in the near future. Tenchi's son will eventually marry Masaki and they will become archaeologists on the request of Washu. He will also have a daughter (little is known about her). The mothers of these children are unknown. However, the picture of Tenchi's son looks a bit like Washu (though some would say Ryoko) and the picture of Ryoko's daughter depicts her as having eyes and facial structure similar to Tenchi's. Further, Tenchi and the girls are living on Jurai by the time he has children.
It has been revealed in the IF doujins that Ryoko and Ayeka will become pregnant at the same time, with daughters who have an extreme resemblance to their mothers, and who will be very close to each other. These two daughters will develop a close relationship with Seina and Kiriko Masaki's son, eventually marrying him. Similarly, Tenchi and Ryo-Ohki will have triplet daughters who will all eventually marry the son of Seina and Amane Kaunaq, and the two daughters Tenchi respectively has with Mihoshi and Noike will both marry the son of Seina and Ryoko Balta. It was also shown that the latter marriage will produce Tenchi with his first granddaughter, through his and Noike's daughter.