you know why season 0/the early manga of yugioh is interesting, because it was so clearly written before it was clear that atem was like...actually atem

the early version of the character was pretty transparently meant to be the other yugi, and only the other yugi, really until somewhere between bakura's first arc and the end of duelist kingdom. but they definitely weren't treated as especially separate characters at first, rather as two halves of a whole, until that point. it was first presented like magical DID more so than ghostly possession.

what's interesting about this is also kind of what's unfortunate about it- it's that yugi's character development also becomes atem's character development. as yugi becomes stronger and grows closer to his friends, so does his other self. yugi's friendships fuel all of atem's actions- atem isn't driven by anything that isn't also driving yugi. he acts like an extension of yugi's will, taking over only when yugi's soul isn't strong enough or physically able to continue.

earlier in this first arc, that tends to happen earlier in the conflicts. as the arc progresses, however, and yugi gets stronger and more confident, atem waits longer and longer to show himself. hell, in bakura's shadow game, it's not until yugi's soul is literally sealed away that atem even appears. as it was written, this arc seems to be the culmination of yugi's first arc- he is now strong enough and confident enough to believe in his friends wholeheartedly, and that faith allows his other self to believe just as much, and ultimately win the game. the two of them are treated like a whole person, like one duelist.

but what you can read into all this, once you know that atem is in fact an entirely separate person, is that this first arc is really about atem first becoming like yugi- about the healing effect yugi inadvertently had on him. when atem first appears, he's this demonic entity with a gambling addiction, meting out sadistic punishments on anyone who hurts yugi or his friends. I think we're first meant to believe this is an alternate personality that the dark powers of the millennium puzzle draw out, but actually no it's this amnesiac ghost in possession of ancient egyptian shadow magic who just got slammed with all the memories of the person whose body he can now possess.

so with hindsight, and you do kinda have to read this into it because atem's character is not explored as his own person until later, but this arc really becomes about how atem finds his chill. over the course of the season he doesn't stop doling out vigilante justice exactly, but he does calm the fuck down. the more he adopts yugi's identity as his own, the more he becomes like yugi- and it softens him. about mid way through season 0 the penalty games become a lot less sadistic and violent- atem goes from burning men alive and torturing them to insanity to making them think their make up is constantly flaking or getting doused in water on stage. there are several opponents he just lets go free, and by the end of the death-t arc he's not only saving mokuba's life, but he's shattering only the corrupted part of kaiba's soul, leaving the good parts of him totally intact. he gives kaiba the chance to heal, something he never did for his earlier opponents. he goes from morally ambiguous to outright good guy. and during the very last arc of this series, he's actively incorporated into the friend group in his own right, even if he is still being treated mostly as the other yugi.

and I think you can and should also infer that this is how he thinks of himself- he has no identity at this point so he totally and completely takes on yugi's and doesn't think twice about it. that would seem to contradict the later reveal that actually no he's a whole other person, but it kinda inadvertently works in its own way. because if the final arc is about atem and yugi separating, this first arc is how they come together.