When Juraian Emperor Azusa visits Earth and attempts to bring his daughters Ayeka and Sasami back home, they will have none of it, wanting to remain there with Tenchi. Still sore over Tenchi's grandfather (and the emperor's own son) Yōshō abandoning Jurai to marry an Earthling, Azusa begrudgingly approves, but only if Tenchi could defeat his chosen fiancé for Ayeka in a duel. This "worthy suitor" was Seiryō Tennan, son of one of Jurai's wealthiest families. Flashy with an exaggerated sense of style (he rigs his own light show and cherry blossoms for special effects), Seiryō sees the events from a skewed point of view, believing the princesses to be confined in a dump much worse than a prison cell. He takes it upon himself to "liberate" Ayeka from her forced labor and thereby the barbarous Tenchi, believing it to be his, or rather, "any civilized person's" moral duty.
Boastful, Seiryō thinks it would be a wise idea for Tenchi to concede - not wanting to "bully the weak" - and beseeches him not to worry as he won't be using his full power. Some time before the duel was to begin, the group makes predictions on whom they think will win: the majority side with Tenchi, Azusa favors Seiryō in "less than a minute", and Washū places an unorthodox bet on Mihoshi.
It turned out Washū was unusually perceptive. Mihoshi's shuttle crash lands in the Masaki family lake before the match begins, swamping the deck of the house and leaving a dazed Seiryō to be washed back into the water. As a result Tenchi won by default, Seiryō losing his chance at arranged marriage.
His "defeat" would be so humiliating for Seiryō that he would develop a personal hatred for all Earthlings, a grudge which would continue on into Tenchi Muyō! GXP. He holds a teacher's position at the Galaxy Police Academy, and immediately develops a distaste for the human Seina Yamada when he becomes a cadet.
Seina becomes Seiryō's nemesis, 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 Kuramitsu found out, and assigned Seriyō to clean toilets... with her niece Mitoto supervising. Seiryō is brash and overconfident, often severely embarrassing himself through acts of sheer incompetence. However, he is also an accomplished fighter, and is quite proficient with a lightsword.
After Amane allowed him to be captured by the enemy in Episode 9, Seiryō learns during Episode 18 the Daluma pirate guild has signed a supply contract with the Tennan family. Seiryō follows his father's orders to assist the pirates and becomes the captain of the Daluma guild's anti-Seina Good Luck Fleet. In a stunning display of idiocy, he names his flagship Unko, which could mean "Luck Bringer" in Japanese... but also translates as "poop".
After fighting the G.P. ship Kamidake II to a standstill twice, Seriyō then challenged Seina to an official duel. However, during the duel, Seina trips on a piece of candy and pulls off Seiryō's belt, exposing his red bikini briefs and costing Seriyō another match by embarrassment.
At the end of the series, Seiryō 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 Kyō Komachi, and proposes to her after revealing that it was her birthday.
