The oldest Kelly child, Bill is starting to become a grown-up, though he has some awkward teen years to finish first:
His ears flared out and were very red, and though he was just out of high school he wore a size thirteen shoe. He tried to hide his feet by scraping one foot behind the other when he stood up, but that only made it worse. (1.3.61)
Eek. We've all been there.
Aside from growing physically, the maturing Bill starts growing increasingly distant from Mick.
"[A violin] is one thing you don't sit down and make – you got to buy them. I thought anybody would know a thing like that. But I figured it wouldn't hurt you if you found out for yourself."
Sometimes she hated Bill more than anyone else in the world. He was entirely different from what he used to be. (1.3.78-9)
Bill is nice guy who's just having a harder time relating to his moody younger sister. He means well, but these days he isn't what Mick needs, or really wants. But he's definitely got our support: Mick's seemingly irrational hatred of her brother makes us feel for the guy. Cut him some slack, Mick!