Theory: most of the differences between Yugi and Atem’s personality are the result of nurture/enviroment, more than they are the result of some inherently different nature.
I don’t feel like letting google’s unholy eyes see me googling yugioh shit looking up a source rn, but I recall people passing around a certain statement. That, according to Takahashi, in some book or interview or another… if not for the Kul Elna massacre, Thief King Bakura’s personality would’ve been closer to Ryou Bakura’s. Or even that it was closer, before that event transpired.
The Bakurae are clearly intended, on some level, to be foils to Yugi and Atem. Even if they kind of suck at it because their writing is a hot bloody mess cough. So, parallels, huh? Let’s talk parallels. If the Bakurae can be said to originate from similar “starting points“… why can’t the same be true of Yugi and Atem?
Yugi has no fucking self-esteem, no self-confidence, and doesn’t reach out to people about his emotions because he’s been severely bullied, probably for most of middle school as well as in high school. He very well could’ve been a lot more confident and self-assured as a young kid, for all we know. But just a few years of intense, persistent, constant harassment and abuse by one’s peers … that’s enough to almost completely destroy a person. Even a formerly self-assured, outgoing person. His only protection from this is Anzu, and later Jounouchi and Honda. But by that point, the damage has already been done.
Meanwhile, Atem had probably never been fucking bullied in his entire life. His position within the context of his society was one of absolute privilege. It didn’t matter if he had the same autistic-as-hell special interest in games as Yugi. It didn’t matter if (as per the height and weight data in the Millenium Book and Gospel of Truth, and despite the actual manga artwork) he had the exact same small, childish, non-masculine physique. If you punch the Pharaoh’s son, you… are probably going to get a very severe punishment for that. If anything, Atem could’ve easily ended up sheltered from consequences that a more ordinary person (like Yugi!) would encounter in everyday life. Mix that in with some fucked-up demon magic postmortem for good measure, and you have a recipe for the cocky, self-assured little bitch whose sheer confidence in his ability to WIN is a danger to himself and others at times. (A trait that’s at its most extreme in the early manga, but can be seen even well into the Duelist Kingdom arc.)
Perhaps if their upbringings had been more similar, they would have been more similar as people, themselves. I think it’s interesting to think about.