Seto's name, like many of the other characters in Tenchi Muyo!, comes from a landmark in and around Okayama Prefecture. In Seto's case, it is not a landmark, but the Seto Inland Sea, which separates Okayama Prefecture from Kagawa Prefecture.
Seto is the mother of Juraian Empress Misaki, the adopted mother of Noike, grandmother of Princesses Ayeka and Sasami, and is a very powerful 'behind the scenes' figure in the politics of planet Jurai and is overall an extremely influential person in the galaxy. A survivor of the pirate raid of the Jurai Royal Space Academy post on planet K-1190 over 5,000 years prior to GXP, Seto was rescued by Juraian noble Ushio Kamiki and adopted into House Kamiki of the Juraian Royal Family. Because of this, Seto retains an intense hatred of pirates. She is married to the Kamiki clan's head, Utsutsumi.
Seto is bonded with a second-generation Royal Treeship called the Mikagami, which is known for her special attack, The Triple Z (AKA the "Genocide Dance"), which literally causes all enemies to stop in their tracks; either due to an Electromagnetic Pulse or some other mechanism, though the effect of this is left to the viewer's imagination.
Because of her political craftiness and occasional violent temper, she is referred to as the "Devil Princess of Jurai," (a name she earned during various battles against pirates) and according to Ayeka, not even her father Emperor Azusa dared to cross Seto, and Ryoko goes on to say that even the ruthless Kagato knew enough not to mess with the Devil Princess.
Seto has a reputation as an unofficial matchmaker in the galaxy. She has been known to arrange marriages either for political reasons such as to cement alliances or just for fun. As her granddaughter Sasami puts it, she "likes to throw rocks into the pond to see how big the ripples are."
Seto had a small, but significant part in the third OVA series (sidenote: she commented that she used to hold Tenchi when he was a baby) It was revealed that she already knew that Prince Yosho had fled to Earth because he had secretly contacted her after he left Jurai in the wake of space pirate Ryoko's attack. Lady Seto is the reason Noike was sent to live with Tenchi; having her pose as Tenchi's fiancee, she commissioned Noike to watch over the residents of the Masaki House on Earth as it has the strongest concentration of power in the galaxy due to all of its inhabitants. Seto arrives at the house for a visit and sort of an "inspection." While there, she bonds with Washu Hakubi, the universe's greatest scientific genius, creating further speculation concerning her connection to Washu's best friend and scientific partner Naja Akara who coincidentally disappeared during the same pirate raid from where Seto was found.
She then borrows Jurai's greatest ship, Tsunami-fune, and used it to throw a party in hopes of bringing antagonists Misao and Mashisu together. Seto later presided over the meeting to punish those involved in the Chobimaru incident and offered to adopt Mashisu so she would be allowed to marry into the prestigious Kuramitsu family until Misao and Mihoshi's mother Mitoto arrived and announced that Mashisu had the Kuramitsu family council's support, which was confirmed when Seto's assistant Minaho announced in a panic that news had broken out on the upcoming wedding.
Seto plays a larger role in OVA spinoff Tenchi Muyo! GXP, and her particular dislike for
space pirates show heavily. Lady Seto first met Seina when his transport ship was under attack by pirates. Seeing how Seina's bad luck could be useful for the Galaxy Police, she agrees to support Seina and makes an exception for him to enter the GP Academy even though he comes from an underdeveloped world, feeling like the academy could use a little chaos once in a while. Some of her supporters didn't like the idea of having Seina joining the GP. Seto had also requested to Headmaster Mikami to have Kiriko reassigned as an instructor at the Academy in order to train Seina to become a man.
Later, when the Daluma pirate guild brought out their good luck fleet and succeed in escaping from the Mikagami, Seto had Seina deal with them. But when Seiryo was made the captain of the luckiest ship in the Daluma fleet, the Unko, knowing the Kamidake II was no match for it in its current state and that it would take too long for Fuku the cabbit, who is the power source for the Kamidake II, to mature, Seto decided to have Kiriko go through her tree bonding ceremony to merge her new Royal Tree Miyuki with the Kamidake II in order to give it the power it needs to bring down the Unko. After using her Mikagami as a test subject for this, the ceremony was performed (with her husband Utsutsumi and Emperor Azusa as witnesses) and then the plan was executed. Seto later arranged a duel between Seiryo and Seina in order to have Seiryo surrender.
Near the end of the series, after Seina brought back the mecha that he found on the Wau planet, Seto held a meeting on the Mikagami, alongside Emperor Azusa and the empresses Misaki and Funaho, to discuss their findings on the mecha with Seina. Seto reported that the goddess Tsunami had told her that the first generation Juraian Royal Tree seed which is used as the mecha's power source is actually one of the two first-generation seeds that Tsunami had given to a person that had visited Jurai long ago. Seto also announced that Seina was now part of the Jurai royal family since the seed had bonded with him.
In the official GXP novels, Seto reveals she has recently regained her memories from before her adoption, and that she was the one to seal her memories. It is revealed that they are the memories of Naja Akara, confirming her identity. Washu is the only other person to be aware of Seto's original identity, presumably having discovered her identity while checking her health during OVA 3, using the names Naja and Seto interchangeably when the two are in private.
Mirror Seto - Kuis Panta
The first time Seto unleashed her ZZZ attack, which stands for surrender or die, she killed so many people it caused Mikagami to be badly injured, because of all the death it caused. Because of this, Seto went into a coma-like state. In Jurai there are agents called shadow warriors (kagemusha), spies that can perfectly copy any person, when Seto went into a coma Jurai needed someone to take her place and make sure spies and other powers did not realize the fleet commander was incapacitated. But in the case of Seto, they went a step further and made a rare "Mirror Warrior." Mirror Seto was in almost every way the exact same as Seto, had the same memories, acted in the same fashion. After Seto recovered, Mirror Seto remained, allowing Seto to be in two places at once, with periodic memory synchronization. When not acting as Seto, she could also act as a normal shadow warrior, a spy able to assume anyone else, under the name, really just an alias, Kuis Panta. And Seto would occasionally assume the identity of Kuis, with Mirror Seto acting in her place when needed. But the entire time they shared the same ego, one truly was the indistinguishable mirror of the other. It gives Seto the impression of being tireless and everpresent. One cam be active while the other half rests.
This arrangement remained flawless until Mirror Seto, acting as Seto, encountered a young Seina Yamada while visiting Yosho and his family. Since then she has kept her memories of Seina to herself, while Seto, having regained her memories as Naja, has found those memories have not been shared with Kuis. This eventually caused Mirror Seto to begin to have the beginnings of her own identity. If Seina had never left Earth there would still be little difference, but with each new experience she shares with Seina and keeps to herself, the more unique her own ego becomes. It is actually becoming a slight danger that outsiders may recognize the difference and the subterfuge will be undone.
When Seina became master of Zinv and bonded with Jinbu, a first generation tree seed, it became possible for Seina to discover the secrets of any lower generation trees (such as Mikagami) and their masters. Although Seina isn't the type to go rummaging for others secrets, Seto and her Mirror decide it best to tell him all about the situation. They tell him that It is possible for the two of them to be permanently separated into two completely separate individuals but it is a very rare thing to happen. It would take a miracle, the discovery of some sort of unknown key to separate them. Before they let Seina go, Kuis asks Seto to move away for a moment and she asks him to give her a name, a fitting name that is a reflection of her true self. As Seina is the one that triggered her ego to emerge, she wants him to be the one to give her the name. When he leaves, Kuis tells Seto that it doesn't matter how long it takes, hundreds or thousands of years, she knows Seina has the key that will free her.
Using the identity of Kuis, she snuck onto Racepshi to join Seina in Renza.
The existence of Mirror Seto is extremely classified, Seina was told that only he and Azusa have been told up to that point, and they plan to tell Yosho eventually (Utsutsumi is NOT mentioned). However, while Kuisa was spending time in Renza, Noike's ship, Kyoko, gained the abilities of a first generation ship and, being curious and immature, she discovered the secret and told Noike, Aeka, Sasami, and maybe others.