Seina Yamada is a good friend of Tenchi Masaki and his parents run a small grocery at the beginning of the series. However, aside from Tenchi, Kiriko Masaki and Kiriko's brother Kai, Seina has very few friends; mostly because he has the worst luck in the universe. Minor disasters often occur in his presence, and anyone near him stays a good distance away from him. Seina's usual (lack of) luck almost ends when a spaceship nearly crashlands on his head at the Masaki residence. Its driver, Galaxy Police Detective Second Class Amane Kaunaq, had meant to present Tenchi a pamphlet for joining the G.P., but mistakenly offers Seina the application instead, under the impression that Tenchi was training the boy.
His family persuades him - via forcibly nabbing his signature and thumb print - to fill out the application, with the hope that Seina would win a prize. When he wakes from sleep for the night, he finds himself aboard a spaceship, recruited to be the newest member of the G.P. Academy. He also meets Mitoto Kuramitsu, the ditzy mother of Mihoshi and cleaning lady extraordinaire, before discovering his friend Kiriko is also a GP officer. From there, Seina is thrown into a plan to stop the Daluma pirate guild from taking over the galaxy.
When Seina's bad luck was found to have an effect of attracting space pirates, the G.P. capitalized on it by placing Seina in command of one of the Decoy Fleet's newest ships, the Kamidake (which included systems designed by Tenchi's sister Tennyo Masaki). However, when the ship was irreparably damaged as a result of an encounter with space pirate Tarant Shank, Seto then presented Seina with a ship designed by Washu Hakubi, which was named the Kamidake II by Seina. The ship consist of the main ship body and a cabbit, similar to Ryo-Ohki, named Fuku by Seina. Fuku differs from Ryo-Ohki in that Fuku serves as a terminal body and more importantly, the power source for the ship, thus is not able to transform into a ship. However, according to Washu Hakubi, it is possible for Fuku to turn into a cabbit girl like Ryo-Ohki... if Fuku wishes for it as she matures.
Later, the ship became a hybrid, when under Seto Kamiki Jurai's supervision, it was fused together with Kiriko's second generation royal tree Mizuki (enabling it to generate Light Hawk Wings for protection). The main pod, which house the royal tree in a Juraian Royal Tree ship, was used for the fusion.
Battles with pirates and resulting revelations
During the time Seina commanded the Kamidake and Kamidake II, he was earning an equivalent to a fleet commander's salary. However, because he was still a cadet, that money was kept in a special account until he graduated and was given what he would normally earn. The interest the money accumulated, however, was forwarded to his family on Earth. It was so much money that the Yamada family used it to build a huge new home, as well as building a huge supermarket in place of their small grocery. While Seina was visiting Tenchi back on Earth, Amane, Kiriko, Ryoko Balta, and Neju, who was visiting at the time, worked at the Yamada's supermarket, where they were worked until exhausted.
It was later in the series when Seina, who was trying to rescue Fuku from rogue elements of the Galaxy Army, was found by a tribe of Wau, who mistake him for their savior and threw him into an ancient mecha, (which bears a striking reselance to Dual!'s Zinv). Somehow, Seina was able to activate the mecha, which he used to save not only Fuku, but also cabbit clones made from Fuku's cells by Tarant, who had allied himself with the rogue G.A. faction. Angered by this, Seina fought Shank and his allies. But in doing this, the mecha was able to integrate itself to the Kamidake II, which was captured with Fuku. And even more startling, the mecha formed Light Hawk Wings, even though it was not bonded with Mizuki.
Afterward, it was revealed by Emperor Azusa and Empresses Funaho and Misaki that the mecha had as its power source a first generation Jurai tree seed, with the seed bonding itself with Seina. As a result, Seina is now third-in-line to the Jurai throne, behind Yosho and Tenchi. Thus, a political marriage between Seina, Amane, Kiriko, Ryoko Balta, and Neju was arranged for political reasons. Seina is initially very disposed against what he thinks is an attempt to force the girls into marrying him, but is later persuaded that they are doing so out of their own free wills.
However, before the ceremony could begin, two incidents occurred. First, Shank had broken into Seina's dressing room, hoping to kill the boy. Luckily, Shank's impaitence gets the best of him and Seina was able to subdue him. Also, Seto's ladies-in-waiting - Gyokuren, Hakuren, Karen, and Suiren - who were actually agents of the Renza Federation seeking to use Seina's abilities to rid their home from marauding pirates, kidnapped Seina into their small ship, and began to molest him in a public broadcast to ensure their non-Galactic Union organization's survival. As a result, the four are apparently included into the marriage, or at least the crew of the Kamidake II, as it patrolls for pirates and illegal activities in space.