Stacey is Cassie's big brother. He's twelve years old and in the seventh grade. He's not too excited about starting school this year, because his mom is his teacher. And even though we never get a look into Stacey's mind, we get the sense that he's doing some growing up of his own.
Stacey has a "code of honor" (as Cassie calls it) (4.195) that dictates how he behaves in relation to others:
Despite our every effort to persuade Stacey otherwise, when Mama came home he confessed that he had been fighting T.J. at the Wallace store and that Mr. Morrison had stopped it. He stood awkwardly before her, disclosing only those things which he could honorably mention. He said nothing of T.J.'s cheating or that Christopher-John, Little Man, and I had been with him, and when Mama asked him a question he could not answer honestly, he simply sat fidgeting nervously throughout the interview. (4.243)
So, even though T.J. got him in trouble, Stacey won't snitch on him. He admits his own guilt and then stops. From this, we get the sense that Stacey is a good guy—but we also worry about him a little. Cassie is willing to break (or at least fudge) the rules. And in this world, being a black kid who won't fudge the rules could be pretty dangerous.
So, what's up with Stacey not liking Mr. Morrison at first? Well, remember that he's twelve years old, so he's right on the border of being a child and being a man. He's not exactly thrilled when Mr. Morrison shows up to help take care of things while Papa is gone. He's "aloof" from Mr. Morrison (4.119), and later comments:
Don't need him here. All that work he doing, I could've done it myself. (4.127)
We're starting to get it: Stacey wants to test drive-being the man of the house, and Mr. Morrison gets in the way of that. But is he really ready to be a man?
By the end of the book, Stacey is a little closer to the "man" side of that border. He's had a bit longer to get used to some of the hard realities of racism and injustice, and part of growing up in Taylor's novel is learning how to deal with these.
For one, he sees that T.J. isn't quite as cool as he thought, and he rejects his brand of superficial friendship. Any dude who will get your Mom fired because he decided to cheat on a test isn't worth your time—that's what Stacey learns. In the end, though, Stacey is willing to risk his own safety to keep an eye on T.J. as the mob attacks him. He's learned Papa's most important lesson: you have to decide when something is worth taking a stand on.
Stacey makes some difficult choices, like rejecting Jeremy Simms's friendship because the price would just be too high if things went wrong. At first, he's into Jeremy:
Actually he's [Jeremy's] much easier to get along with than T.J. [...] And I s'pose if I let him, he could be a better friend than T.J. (7.106)
Papa sets him straight on this, though:
You see blacks hanging 'round with whites, they're headed for trouble. Maybe one day whites and blacks can be real friends, but right now the country ain't built that way. Now you could be right 'bout Jeremy making a much finer friend than T.J. ever will be. The trouble is, down here in Mississippi, it costs too much to find out. (7.109)
And we know that—unlike Cassie—Stacey gets it. He literally shuts the door on a friendship with Jeremy when he closes the lid on the treasure box in which he stores the wooden flute that Jeremy has made for him (7.111).
Mission complete: one fully formed adult.