Airen is a supporting character who first appeared in the Renza Arc of the GXP Novels and made her animated debut in Tenchi Muyo! GXP Paradise Starting. It is heavily implied, but not yet confirmed, that she married Seina at the same time as Jovia Jovis.
Background
Her full name is Airen Kariyuuku Kaou. She is an elf and a princess, low in succession, of the Kaou Kingdom. Souren is her older brother. If Seina did marry Airen, it would mean that he has married a princess or queen from each of the royal houses of Renza.
Airen is first introduced in a flashforward story told in the first chapter of the first novel of the Renza arc of the GXP novels. She was a Renza Military Officer, the head of maintenance, specializing in the Knight Calvary, on the single desert spaceport on Barrium. Her commanding officer was a selfish, cowardly, noble who liked her around for the prestige of her role. However, she actually has a rather rough demeanor, appropriate for a glorified mechanic who can efficiently fix giant machinery. Rough as she is, she is still a beautiful woman. Both her character and beauty cause consternation to her long suffering lieutenant, Nagato.
On the base, she meets an interesting young recruit, who we come to realize is Seina. Seina had originally traveled to Barrium after his fight with HIMC to fix the battered Zinv and remembered that, according to Souren, a friendly person could be found there. But for whatever reason, he wasn't revealing his identity to her at this time and was working on an independent plan to return to space as a recruit. Airen could immediately she the talent he was trying to conceal and kept an eye on him. A couple weeks into the training, a fleet of pirates attacked the base, to loot their supplies. The commander retreated with all of the "priveleged noble" cadets and left the rest to be sacrificed. But Seina, in a low-powered practice mecha, almost single-handedly defeated the entire raiding party. Even the defeated pirates were impressed, but the commander tried to scold and punish Seina for showing him up. When Seina answered the commander's comments with a request for a duel (which he could not refuse), Airen realized that she had fallen in love with him.
The details of the rest of the Renza arc have not been recorded at this time. However, after whatever events did happen, Airen and Souren, as royalty and members of the military, were helping the diplomatic mission iron out the details. But after a while it became evident to them that Kiriko and the others were more than capable of doing the work themselves, so they used magical transportation by spirit thread to travel to the subspace magical environment on the Kamidake. Once Tsukiko arrives and starts to develop a magical organ, Airen assists her in her magic lessons.
At different points in the 5th Season of Tenchi Muyo! Ryo-Ohki it is implied that Airen has married Seina by that time. Most notably, Jovia explains that she married Seina as "bench-warmer wife" and "a plus-one to a great beauty." Knowing that The two were on the same planet, and such a marriage would probably secure Seina's safe anonymous passage back into space, it makes the most sense that the "great beauty" is Airen. And when Airen and Jovia get together in Tenchi Muyo! GXP Paradise Starting, they are shown to be very close and the types that enjoy launching schemes together.