One thing about Kaiba Seto I'm still perplexed about the Yugioh anime never having explored more of despite it constantly being relevant to his characterization towards the Gang, is that back in the orphanage, 10-year-old Seto had to quickly learn to view his same-aged peers as threats because some of said other orphans thought it was funny to take their frustrations and feelings of being powerless and abandoned out on 5-year-old Mokuba who already was drowning in depression after their parents died and their other relatives ditched them:
That is such a crucial detail about Seto and the way he had to learn to view his peers. As threats he has to be ready to fight because all of them could suddenly prove themselves to be a danger to him and especially Mokuba.
So his instincts tell him he can't let anyone in and befriend them, even if they are kind, helpful, and friends with Mokuba right now. 'Cause what if he let's his guard down and becomes friends with them, too, and THEN they show their true colours because they needed him to stop being cautious first? And Seto knows exactly that if the Gang were to attack one of them, it wouldn't be Seto. It would be 5-years-younger Mokuba first, whose trust they've gained by now, because he himself has been keeping himself so safe and inapproachable to everyone around him that the Gang knows that the best way to hurt him is hurting Mokuba. And at thing point, they would KNOW how to hurt them best.
So he can't stop being cautious. And especially not if the Gang are the first people in so many years that Seto would even start daring to hope for that making friends might be a safe thing to do again.
Seriously, how tf did this never come up to be given any kind of development? The show had the Gang not exactly rarely question why Seto is still so hostile and not letting his guard down around them and why they even bother. But the show also had very little interest in giving Seto actual development for that flaw, or even just EXPLORE what his problem with them still is.
Which is really weird considering how much this show LOVED Seto. So one would think exploring WHY we were in this eternal stalemate between Seto and the Gang would have at some point been declared a priority over, for example, completely inconsequentially inventing Siegfried into Seto's backstory as some weird peer he met like ONCE, but nothing really happened there, so it also did pretty much nothing for Seto's character (if I'm wrong about this, feel free to enlighten me)
Cause in comparison to that, the information that Seto protected Mokuba from bullies Seto's age in the orphanage is one we got in season 1. So it's not like this is backstory they only added in later on and couldn't do much more with because of it. They could have explored Seto's hostility towards making friends the ENTIRE time from s2 onwards. They just didn't do it and I find that baffling to this day.
