Cassie says Stacey feels responsible for T.J. because he realizes that he is vulnerable.

Cassie says she doesn’t really like T.J., and neither does their brother Little Man. Stacey is much more willing to accept T.J. and his underhanded pranks and shenanigans, such as telling his mother lies so his brother is punished instead of him.

When T.J. shows up at the Logan house in the dark telling them he is in trouble, Stacey suggests he go to R.W. and Melvin. T.J. tells them he is the one who got him into trouble. 

I . . . I’m sick, Stacey. I gotta get home ’fore my daddy wake up. . . He say I stay ’way from that house one more night, he gonna put me out, and he mean it, too. He put me out, I got no place to go. You gotta help me (Chapter 11). 

T.J. tells them what happened: he broke into the store with R.W. and Melvin to steal the gun and attacked the store owners. This is why there is a lynch mob after him. Cassie doesn't want Stacey to get involved, but she knows he will try to help T.J. because he always tries to help T.J.  

As far back as I could remember, Stacey had felt a responsibility for T.J. I had never really understood why. Perhaps he felt that even a person as despicable as T.J. needed someone he could call “friend,” or perhaps he sensed T.J.’s vulnerability better than T.J. did himself (Chapter 11).

While T.J. likes to pretend he is tough, he actually isn't as tough as he acts. Stacey has always realized this. Now that T.J. is in really deep, he still comes to Stacey when he needs help. He has put his entire family in danger.