Now, I'll start off by saying that Theoris turned out very different from her predecessors (OCs who were meant to be paired up with Kaiba but got remade again and again until scrapped altogether and Theoris was born), not just in looks but also in personality: the OCs that came before had incredibly powerful senses when it came to supernatural things and were strong believers of it, as well as being fairly competent in their use of InsertMillenniumItem here. They embraced their destiny.
Theoris on the other hand, is cut from a different cloth than these. Where the former embraced the destiny they were given, Theoris is actively fleeing hers and while bearing the Macguffin Item known as the Sacred Menat, she refuses to learn how to wield it simply because it's tied to Destiny. It's not until post-Battle City that she's finally motivated to start taking steps to mastering the use of the Menat. The reason: protecting Seto. If watching his back includes keeping an eye out for supernatural threats, so be it.
On to the ship itself.
From Seto's end of the pairing, his attraction stems from when he interviews Theoris for a job at Kaiba Corp (she's applying for a secretary position). She's given five minutes to prove why she should be hired and during those five minutes, he presents her with a chess puzzle where White is under threat of being checkmated. Of course, the way the puzzle is set up, there is no outcome where White comes out the victor: it is unwinnable.
It doesn't stop Theoris from trying to solve it all the same. Her first move is to move a knight to a position where it can intercept any of the pieces coming for the White King. This results in her getting six more turns to delay White's defeat. In her second and last move, she decides to be bold and aggressive by moving in a bishop to capture the Black King, as the path is clear. Of course, due to the puzzle being unwinnable she still loses...but it is enough to attract a certain CEO's interest.
For Theoris, her attraction is initially admiration and respect for Seto Kaiba. While not aware of the whole story, the way he took charge of his own fate strikes a chord within her and gives her hope that, even entrenched as she is in the fate she's running from, she can do the same. There's also the fact that he happens to be her type: arrogant, and capable of proving to the world why he's within his rights to be so while the Gods look away.
Now, both sides of the ship have a mutual attraction for one another...The frustrating part is that neither is the best at opening up about their feelings (though Theoris has an advantage over Seto due to having been in a relationship before) and both believe it's just a pipe dream. It takes a trip back in time to 1486 Germany (where they meet their DotR incarnations)--which Seto treats as a coma-induced dream throughout most of their stay--for the two to come clean about their feelings. Seto's treatment of the experience, in fact, is the source of origin for their shipping label: Dreamshipping.
As for why I'm writing about these two? Well, turns out rping their dynamic is a lot of fun, if at times frustrating (in part because PercentiledOne nails his portrayal of Seto Kaiba right on the head and Seto Kaiba has a tendency towards frustrating people). The vibe they give off as a pairing (and this includes their DotR incarnations) is also interesting: you get a sense that these two can be rather selfish in their love for one another yet still be considerate and caring. It's also fun to wax hypotheticals about their reactions to certain situations.
In short, this is a very fun pairing and I look forward to exploring them further. =)