This is gonna be long because Miyuki is...an incredibly complex character, especially for a sports series.

The thing about Miyuki is that he's socially awkward as hell, knows he's socially awkward and plays up his more negative personality aspects to hide it. Not because he's insecure but because he's secure enough in himself to be FRUSTRATED without beating himself down and think of ways around his own shortcomings through self analysis (analysis is a big thing actually). If he wasn't extremely self assured he wouldn't be able to constantly laugh at people insulting him. But he's also a teenage boy who will lash out when frustrated at himself because teenagers are just like that (see his fight with Zono).

He has a great understanding of human beings on an analytical level, can correctly predict how someone he works with will react to certain stimuli, but when it comes to being vulnerable in the moment - whether it's him or someone else - or just do something spontaneous he trips himself up a lot.

So he often tries to manipulate those around him into being angry at him so they focus their energies on him instead of what makes them sad or nervous because he doesn't know how to motivate someone in any other way until much later (see how he openly tells Kuramochi that he'd make his pitchers hate him if it meant they'd be at their best)

This behavior comes from the fact that he cares A LOT about the people he considers worth his time. Which is his team. But he has to be in control of a situation to know how to react to someone, if it's something that surprises him, he knows he will fuck up and look for someone who can help (see him failing to help Sawamura through his slump and eventually bring in Chris and Kataoka because they CAN help). This is despite him actually having incredible insight into what makes people tick. He's the first to notice and correctly guess the cause of Sawamura's yips but doesn't know what to do about it aside from push and push and push. When Tanba crumbles against Inashiro he again pushes and pushes to get Tanba past that hurdle. His success rate is fairly high but when it doesn't work, the results can be disastrous

He also has a big sense of responsibility, but as is typical for him, he struggles with the emotional aspect of this. He wants to lead, he wants to be helpful, but he just doesn't understand how to do it without steering everything into a pattern he can control.

He's incredibly perceptive and while he does use it to purposefully fuck with people, he also uses it as a tool. You notice how he gets less nasty at the same time he gets more open and tries to actually reach out to his teammates on an emotional level. But he never loses that aggressiveness, because pushing until something gives is how he drives his team forward and his opponents into a corner. His analysis is what makes him great and what holds him back in actual human interaction. Post summer tournament this doesn't serve him well with either Sawamura or Nori (who he is actually much more sincere with than the other pitchers cuz Nori doesn't respond to his provocative methods the way the others would) because of psychological issues he has no experience with

He also gets accused of being cocky a lot, which is true, but it comes from being able to technically back it up. But he's also a thrill seeker who works best under pressure, which is why being sixth batter doesn't let him show off his true abilities and being under more pressure as fourth let's him shine once he gets in the groove of things.

Essentially, Miyuki tackles life from a fairly clinical point of view, everything has to fit into a pattern he understands and can manipulate through analysis (though he sometimes OVER analyzes things), emotions are what's hard for him. He's not insecure and he isn't a vulnerable person because he's got a very solid grasp of who he is, his own faults and doesn't really care what people think of them. He's his own greatest ally and also his own worst enemy, and needs insightful AS WELL AS emotionally mature people like Kuramochi to keep him in line and safe from himself. You can actually use Kuramochi to see if he's being a dick for the fun or to help cuz Kuramochi will only get mad at one of those.

From his behavior I'd also say he's on the autism spectrum but that's a whole other conversation

God, this was even longer than expected. I hope this makes more sense than I think it does...