The way Duel Monsters work is something I've been working out a lot recently, and I like my current theory very much. I think there's a lot of room to play, and anyway, it suits the setting. If this stuff showed up in canon, I don't think it would be out of place at all.
I hope so, anyway.
.
"You met my mother and father over the holidays," said Henry. "Kay's grandparents."
"I did." Seto nodded.
Henry was seated on a folding chair in one corner of the staffroom, rubbing his hands and looking anxious. He ran his hands through his hair, scratched at his beard; he looked like a suspect in a crime drama, trying and failing to keep his cool during an interrogation.
Which made Seto, who stood in the center of the room with his arms crossed over his chest, the detective. Kisara might have been his partner, considering just how grave her expression was. She looked like she was preparing for a bloodbath; like she didn't expect to survive the day. The only thing keeping her from punching through the wall to fight against the remainder of her enemies outside, with her own hands, was Mokuba; he stood sentinel beside her, patting her arm.
"When I was young," Henry went on, "around nine years old, I think. I was exploring a field near where I grew up. I'd skipped school that day, on a whim, and I fully believed that the . . . thing I saw that day, stalking my shadow, was punishment for disobeying. I was so sure that the Devil was chasing me that I told my parents as soon as I got home."
Seto grunted. He couldn't pretend to understand where this story was going; from the look on her face, Kisara had no clue what was going on, either. The only person who didn't seem completely lost was Zelda.
"I didn't think about punishment. I wasn't worried about it. I just had to tell someone what I'd seen. I wanted to make sure I wasn't losing my mind." Henry looked Seto in the eye. "Do you know what my mother told me?"
"Mm?" Seto offered.
"She told me that I had seen a monster. Not the Devil, but a monster all the same; a creature from a place outside of Heaven, at the shores of Hell." He turned and looked over at his daughter. "She told me that these monsters, sometimes, would choose human champions. Offer their power to them, strengthen them and protect them, like in mythology."
Kisara watched her father, oddly calm now; she'd come to understand something.
"One of the monsters chose you, didn't they, Kay?"
Kisara nodded. "Yes," she said.
"Which . . . which one?"
Seto reached into a pocket, drew out a Magic & Wizards card, and held it up for Henry to see. "The Mistress of Furious Lights. The White Mother. The Queen of Dragons." He paused. "Though, that's not quite right. I am her champion, Mister St. Vincent. Your daughter is her . . . avatar. Her aspect. The queen sleeps in Kisara's blood."
Henry's eyes widened; he looked upon his eldest daughter in a new light.
"This is why you're so calm, isn't it?" Henry asked, turning his attention back to Seto. "Why you aren't frightened. You both have the strength of a dragon. The dragon. What could possibly stand against you?"
"Plenty could stand against us, Mister St. Vincent," Seto said, "but that doesn't mean we will not prevail. You know enough of the veil behind reality, how magic functions, that you should know what I mean when I tell you this. Even if the queen can't defeat whatever stands before us, I've been chosen by another. Whatever she can't defeat, he can."
"Who?"
"Obelisk."