I first came up with a lot of this world-building for "Lightbringer," part of my "Paved with Good Intentions" series, but it's relevant here and felt like an important thing to cover again, as it's become woven into my personal YGO canon.
Which I guess would make this an AU, but … I dunno.
Definitions are weird like that.
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Seto didn't pray; this surprised absolutely no one when they found out. The reason why, however, often did. It had nothing to do with religion, or even his general antagonism toward gods of all stripes; that was ancillary at best. While it was true that Seto had little patience and less tolerance for supernatural forces—benevolent or otherwise—the actual reason he abstained from prayer was because he couldn't stand to place his personal wishes on that kind of pedestal.
"What I want out of life is relevant to so few honest discussions that, statistically, it's irrelevant," he often said on the subject. "Even if I did think to ask for divine intervention, there are plenty of people across the planet far more deserving of that attention than I. Do I look like I want for much? I don't need the help. I've never needed the help."
This tended to end the discussion for most.
Kisara was not most; she was, so far, the only person to push past this argument.
"I remember many heartfelt promises from a boy who looked like you," she said. "Promises to prove worthy of my power, if only I would lend it to him. I wonder if you mean to honestly argue to me that that is not a prayer. If you do, I must wonder at the usefulness of your definition."
"If I were to agree with your stance," Seto said, sharply, "then I would say that you are one of the only figures in that boy's life to prove worthy of any such grandstanding. Which is more than I can say for most of the authority figures this boy has dealt with. If we were to agree that I know this boy, anyway. Hypothetically."
"Hypothetically," Kisara repeated, smirking so that one fang was visible.
". . . How did you hear me?"
Kisara's smirk softened to a smile, and her eyes turned far away. "I spent much of my time asleep," she said, and they both understood that the dragon was speaking now. "It is the way of many powerful monsters who dwell in the Barrier. We conserve our energy so that smaller monsters can make use of available resources as would suit them best. We have little need for such things."
"The Barrier is . . . the afterlife," Seto said.
"If you were to press me to call it anything," Kisara said, "I would say it is Purgatory."
Seto nodded. "So," he said, "you heard me in your dreams."
"Something like that," Kisara said. "In the Great Arcade, where the High Court conducts its business, there sits a scrying pool. Led by the magicians, a prospective monster will peer into the pool and seek out a human champion. In this way they will prove their worth, that they might ascend into the Golden Fields of Order."
"Champion," Seto murmured.
"For centuries," Kisara went on, "it was an open offer from Black Magician himself, head of his order, to assist in my reading of the pool to seek out a champion of my own. It was only when your words reached me, on the winds of your conviction, that I accepted the offer. So it came to pass that I found you."
"When did you decide to accept me as your champion?" Seto asked.
Kisara quirked an eyebrow. "When did you decide to become your brother's protector?"
"As soon as I lay eyes on him," Seto said.
Kisara locked eyes with Seto and said nothing else.