Humans inherently see themselves as the main character, mostly to feel special and different from others. They see other people as NPCs that don't have any real consciousness and are just cogs in a machine, while they, on the other hand, actually have consciousness and are real.
This is commonly known as main character syndrome. People with main character syndrome believe that the world revolves around them, but deep down, they just really want validation and attention from other people, despite viewing them as NPCS but Shidou doesn't care about validation from other people.
He instead sees himself as the pinnacle of humanity, and in turn, since he is the peak, why would he want validation from people he considers lesser? He is, in his own word the main actor of this world, and all the other people are the supporting cast members.
What would make someone turn out like this, you ask? Well, it's pretty simple in a society based on looks and power. Shidou was extremely fortunate to be born with both. He stands at a giant height of 6,3 at just 15 years old, with the average height of a Japanese Man being 5,7. Ever since he was 12, he has constantly towered over everybody, including his dad and mom.
He was also gifted naturally with an unreal level of looks, despite never trying skincare or any of the sorts he always had a nice, smooth, glossy skin. He has silvery-white hair that falls over his forehead, dark brown eyes, a sharp jawline, and high cheekbones.
This granted him the power of the halo effect, which gets him special treatment from, of course, girls, but it doesn't just apply to people who are attracted to him; it also applies to anybody, really, who would find him attractive despite not being attracted to him like dudes.
For most of his childhood, he has been seen as an angel that could do no wrong, so because of this, he has barely learned to face the consequences of his own actions and act how he wants to act without stopping to think, could this potentially hurt someone? or their feelings.
Now, for his power, height grants you a presence in someone's mind unconsciously, for example, if there were two guys you saw on the bus, and one was of average height and the other was tall, you would most likely remember the tall person. Presence can be seen as having power somewhat with Intamindation, but Shidou was also born with something that even surpassed this.
He has an almost superhuman level of Time Perception. Time Perception basically means how fast your brain can perceive an event. The normal human reaction time is around 200-250 milliseconds, while elite fighters and athletes can react around 120-180 milliseconds. Shidou, while in combat or if his body feels like he's in danger, can react around 10-90 milliseconds.
Because of this, the rare moments where somebody would stand up to him like a kid that he's fucking with, would get their ass kicked before they even knew what was happening. It was brutal, honestly, and due to all of this combination, it created a personality, a personality that believes he was the king of the world.
Shidou lived by the philosophy of Radical Individualism; he isolated himself from his peers and pushed himself in boxing every day to reach his full potential, but he had no ambition to pursue a career in boxing; he simply wanted to get better and beat people up so he didn't have to face repercussions. He believed that the sole purpose of humanity and millions of years of evolution was so that he and he only could be brought into this world.
He was the chosen one, but in a world like Jujustu Kaisen, where those who chose to hold onto their humanity will always be weak, and those who discard their humanity in the pursuit of power and jujustu will obtain the title...
Of The Strongest.
Would Shidou, in his pursuit to prove to himself that he is the chosen one and the strongest, discard his own humanity to obtain such a title?
Could he surpass Satoru Gojo, the Honorod One?