There was a reason that Kaiba Seto was vocally adamant that magic didn't exist, whenever the subject came up. Oh yes, he saw all the things that Yugi and the others did, and he recognised it for what it was, but he still refused to say that magic existed.
Even though he knew better than all of them that it did, and that there were still witches and wizards practising spells and such in the world today, though it had changed over the centuries.
Seto had been adopted into the Kaiba family shortly after turning fourteen. He'd gone to the goblins and they'd discovered the horcrux in his scar, removed it for him, then helped him change his appearance completely, given him a language lozenge, and he'd run away from Magical Britain. If they weren't going to give his godfather a fair trial, he wanted nothing to do with the idiots.
Sirius had done the same thing actually, but was enjoying the Caribbean, rather than Japan. They wrote, but the man needed the sun more than Harry had needed the man in person. Besides, being adopted by Mr Kaiba had turned out to be a very good thing. It meant that he had a better idea of what to do with all the money that his parents had left him, and he knew how to take control of his godfather's money as well, since he was still technically a convict on the run, even if he wouldn't ever be found now.
Seto never said that magic was real because, if he did, the local Ministry would haul him off for telling muggles about magic. Never mind that they knew about an ancient Egyptian magic already. They had no idea about the modern magical community, and if a wizard told a muggle that magic was real, then that was breaking the law.
So regardless of Yugi, Ryou and Malik, and even his own little adopted brother, Kaiba Seto declared firmly that he did not believe in magic, and that magic did not exist.
However much he wanted to show them what he'd been learning from the books he'd been owl-ordering since he left Hogwarts.