Technically, Michael is the main character.
Anyway, I always felt that if Michael’s resilience was not supernatural but physiological, I figured it was a few things. I think he’s got probably a few tumors in his body, likely his brain, pressing on glands in places that interfere with his body chemistry. I think it’s suppressed his ability to feel pain, and is sending out huge jolts of adrenaline and testosterone and cortisol and things like that, amping up his aggression, his strength, and combined with his inability to feel most instances of pain, would allow him to take such sever bearings and just keep going, not really knowing how badly he’s been injured. And as for his surviving so many gunshots, I chalk that up to his healing being amped up higher than a normal person’s, combined with really shitty aim on behalf of the shooters; they just never happen to hit anything vital. Notice how every time something that could happen to destroy his head, or his entire body, is always narrowly avoided?
And this could apply to any version of Michael in any of the continuities. Think of it like he’s one of the “mutant” people in M. Night Shamalan’s Unbreakable trilogy; he’s got “super powers” due to medical conditions affecting him in ways that are normally never detected and appear to be supernatural if viewed outside the context of being medical in nature. But in Michael’s case, it’s screwed up his mental state as well, making him violent beyond all reason. Imagine if that hadn’t happened? If he had never gotten it in his head to kill his sister and then keep going from there, would he have ever even known anything was different about him?