Disclaimer: I do not own Seto Kaiba, Mary Sue, or anything associated with Yu-Gi-Oh!
Author's Note: This is not intended to defame or bully any one particular author. Most of us who ship Seto Kaiba with an OC are guilty of something in this story, so this is intended to poke fun at everyone.
Mary Sue Abridged
Seto Kaiba collided with Mary Sue at high school and/or hired her to work for him. Despite Mary Sue's shortcomings, he found her more attractive than all his other fangirls because she was not constantly drooling over him. In fact, she never imagined that he would be interested in her and never took him for granted. She had very little confidence in herself. Mary Sue liked Seto Kaiba as a person, not for his money or good looks. She babysat his little brother Mokuba on a few occasions, and that made Mary OK in Seto's book. Then, Mary got mugged and/or raped by some thugs on the street. These thugs were rivals of Seto Kaiba. The thugs were most likely working for Bakura, Marik, Duke Devlin, or occasionally and evil version of Joey and Yugi. If Joey and Yugi weren't the thugs, Mary Sue was good friends with the geek squad and Seto tolerated them because of her. Anyway, back to the mugging/rape. Seto saved her at the last minute and took her to his mansion where he or one of his servants undressed her and tended to her wounds.
Despite the fact that Seto is a CEO who has a company to run, he still works out and kicks anyone's ass in a physical fight. He also knows how to treat broken limbs. Yeah, that's because his step-father abused him. Oh, and he has scars on his back and/or chest where his step-father beat him. He's really self-conscious about it, but opens up to Mary because they've both been through traumatic experiences. No, he has never heard of a help hotline or was too afraid to call it because of what his step-father might do to Mokuba. So, then it comes out that Seto murdered Gozaburo by pushing him out the window. He didn't commit suicide after all. Mary Sue understands everything that Seto has been through because she has been through something very similar. Now we have a passion kiss, but they are both still insecure and don't know if they can handle a serious relationship.
Enter Mary Sue's rival. Her name is Valorie, Vivian, Veronica, Vanessa, Vicki, Tea, Ishizu, or some rich bitch (Unless Mary Sue is also a CEO, in which case the rival is an unfortunate gold digger.) Rival manages to kiss Seto and/or spread rumors that drive a rift between Mary and Seto. Mary does something to expose or humiliate Rival. Rival scampers away with her tail between her legs. Romantic dates and fluff between Seto and Mary occur. If Mary was not raped, she loses her virginity to Seto. Finally, Seto proposes. Mary Sue gets cold feet. Rival may return for a while. Millennium items go haywire. Of course, love prevails in the end and they get married. Then comes a baby in a baby carriage.