Seiryo Tennan is first introduced as an arrogant and foppish son of a very prestigious family, and prince. Emperor Azusa offered Seiryo the hand in marriage of Ayeka in order to entice Ayeka back to Jurai. When Ayeka refused to return and marry Seiryo, Azusa then proposed a duel between Seiryo and Tenchi, but just before the battle could begin, Mihoshi's shuttle crashed landed on the lake, washing Seiryo into it. As a result, Tenchi won by default
After his duel with Tenchi, Seiryo became an instructor at the Galaxy Police Academy. He then grew a strong distaste against Earthlings in general, causing him to mistreat Seina when he enrolled in the G.P. Academy. His intense hatred and rivalry towards Seina is a major drive plot and focus in the series. It is also revealed that Seiryo was a childhood friend of Amane and previously dated her. Amane, however, regarded Seiryo correctly as an idiot, and did her best to ignore him.
Seina becomes Seiryo's nemesis (though ironically Seina himself seems to be unaware of this and thinks of him as his instructor with his best interest in mind, constantly foiling his endless idiotic plots for revenge (mostly through Seina's streak of bad luck). Having rebounded from his missed shot at Ayeka, Seiryō becomes infatuated again by childhood acquaintance Amane, despite being frequently and violently rejected; her attachment to Seina only serves to deepen his hatred for the young captain, so much so that in Episode 6, Seriyō holds up the necessary body augmentation that Seina needed to at least survive life at the academy. However, Mikami found out, and assigned Seriyo to clean toilets... with her niece Mitoto supervising, much to Seriyō's chargin and reluctance.
After Amane allowed him to be captured by the enemy in Episode 9, Seiryo is held captive by enemy pirates for an unknown specified amount of time. He is placed supervision and under guard by Kyo Komachi, who expresses her constant annoyance to Seiryo's brash and overconfident incompetence.
He is confined in a jail cell with comfortable riches, gourmet food and luxurious accommodations yet still called it harsh conditions, still driven by the illusion that his family will attempt to pay a hefty ransom to ensure his freedom. Yet he does admit to himself he knows his father loves his money more.
In one episode, when he finally decides to escape, he is later recaptured and imprisoned this time in a jail cell, where it is later revealed that Kyō had arranged Seiryo to escape in order to use him as a decoy to accomplish her goals, which was the freeing of the families of Ryoko's crew from the clutches of Tarant Shank.
During Episode 18, Seiryo learns that the Daluma pirate guild has signed a supply contract for "good luck charms" with the Tennan family. Seiryo reluctantly follows his father's orders to assist the pirates and becomes an ally to Daluma.
Seiryo undergoes a series of tests to earn his place to become the captain of the Daluma guild's Luck Fleet's flagship, to whom which the victor would handle the flagship's final construction stages and earn command of the flagship. Seriyo is released from his jail cell and briefly and easily defeats the pirate Shank in a lightsword match, but later loses the next round to Seina's former three crewmen in a game of rock-paper-scissors.
When the three crewmen use up all of the suitcase of money given to them to complete construction of the Daluma guild's Luck Fleet's flagship (which they all spent on partying all week with the pirates), Seiryo agrees to help the three crewmen and takes command of the mission, using his winning lottery ticket money and his crewmen's assistance to buy a hefty amount of "good luck charms" to be used on the flagship, only to run out of many again.
Seiryo goes on a number missions to re-obtain money again to complete the construction to the flagship, but frequently encounters a streak of bad luck and mishaps throughout each occasion, first being separated from his three crewmen to being stranded in a snow mountain with a beautiful unknown young woman, whom she saved him from a blizzard.
Seiryo spends an unknown amount of time with this young woman, entering into a brief romantic relationship with her and having four sons with her, before being forced to abandon her and his sons in order to continue his mission. When Seiryo is rescued by Kyō from being adrift in deep space after escaping from a black hole, Seiryo encounters a lucky vase in Kyo's living room, his "lucky meter" goes up and he believes that the vase would finally complete the construction of the lucky flagship and adds the lucky vase to the flagship.
Once construction of Daluma guild's Luck Fleet's flagship is completed, Seiryo becomes the captain of the Daluma guild's anti-Seina Good Luck Fleet's flagship, with Kyo and the same three crewmen at his side. In a stunning display of idiocy, Seriyō is forced to name his flagship "Unko", after all the other ships in the Daluma guild's anti-Seina Good Luck Fleet's flagship had taken many of the "perfect good luck names". ("Unko" which could mean "Luck Bringer" in Japanese... is also translated as "poop".)
After fighting the G.P. ship "Kamidake II" to a standstill twice, Seriyo then challenged Seina to an official duel. However, during the duel, Seina trips on a piece of candy and accidentally pulls off Seiryo's belt, exposing his red bikini briefs and costing Seriyo another match by embarrassment.
At the end of the series, Seiryo works as the head waiter at a space station behind Earth's moon, where Seina's wedding was being held, as the station was owned by the Tennan family. While there, he encounters space pirate and former captor Kyo, where he gives her a cup of tea and has an intimate conversation with her, before purposing to her after revealing that it was her birthday.
Seiryo has shown to have matured into more of a man. He also shows that he has finally gotten over his infatuation over Ayame, although still may hold his rivalry and grudge towards Seina.