concussions. Rusty is aware of being forgetful. That happens to everybody, but Rusty's situation is more severe, and he knows that it is triggered more often when he is alone.

A bit later in the chapter, Rusty tells about the time he was put in solitary confinement. Readers can figure out that the experience was bad, and Motorcycle Boy explains later in the book that Rusty is likely afraid of being alone because of something that happened much earlier in his life.

Anyway, he left you alone in the house for those three days. We didn't live where we do now. It was a very large house. She abandoned me eventually, and they took me back to the old man. He'd sobered up enough to go home. I suppose you developed your fear of being alone then.

A third quote about Rusty's fear of being alone occurs after Rusty regains consciousness. He wakes up and realizes that nobody is around, and he panics.

I wasn't out too long, just long enough to scare Steve into looking for some help, so when I came to I was laying there on the roof by myself. I fixed that as soon as possible, almost running to the roof door.

You do not indicate whether the quotes need to address a particular character or theme.  The following quotes describe a central character, Motorcycle Boy, and the theme suggested by the title, Rumble Fish.

"Motorcycle Boy...didn't belong - anywhere - and what was worse, he didn't want to" (Chapter 8).

Rusty-James's father describes Motorcycle Boy, saying,

"...every now and then a person comes along who has a different view of the world than does the usual person.  Notice I said 'usual', not 'normal".  That does not make him crazy.  An acute perception does not make you crazy.  However, sometimes it drives you crazy" (Chapter 11).

He goes on to explain that Motorcycle Boy

"...is merely miscast in a play.  He would have made a perfect knight in a different century, or a very good pagan prince in a time of heroes.  He was born in the wrong era, on the wrong side of the river, with the ability to do anything and finding nothing he wants to do" (Chapter 11).

Motorcycle Boy is fascinated by

"Rumble fish...they'd kill each other if they could...if you leaned a mirror against the bowl they'd kill themselves fighting their own reflection" (Chapter 10).

The rumble fish represent the boys, trapped in an environment of poverty and hopelessness.  Motorcycle Boy tries to free the fish to see if they would do better in the river, a more wholesome environment, but he falls short of his goal:

"...when they turned him over...he was smiling, and the little rumble fish were flipping and dying around him, still too far from the river" (Chapter 11).