"Dead. Or in a crazy house. Or married. I think she's married and quieted down and maybe right in this very city" (1.31).
This is the narrator's answer to Joe's question about Holly's whereabouts, and it's pretty significant that his first instinct is to think that she's dead or locked away somewhere. It seems she's been rather successful in convincing the world that she is meant to spend her life alone.
Her cheek came to rest against my shoulder, a warm damp weight. "Why are you crying?" She sprang back, sat up. "Oh, for God's sake," she said, starting for the window and the fire escape, "I hate snoops" (3.58).
There's a brief moment when Holly lets down her guard and invites someone in, but as soon as the narrator presses her she goes back to isolating herself.
Like many people with a bold fondness for volunteering personal information, anything that suggested a direct question, a pinning-down, put her on guard (3.23).
Holly's unwillingness to answer direct questions reflects her active move to isolate herself from other people. This way she's always in control.
"We sort of just took up by the river one day, we don't belong to each other: he's an independent and, so am I" (4.52).
Holly won't even allow herself to form ties with a cat. She remains completely isolated from anything that could represent personal attachment.
We wore the masks all the way home (7.6).
This is the scene when Holly and the narrator steal the Halloween masks from Woolworth's, and it carries a great deal of symbolic meaning in relation to the idea of isolation. There are a lot of masks Holly hides behind, and this allows her to maintain the isolation she seems to need so dearly.
She talked of her own [childhood], too; but it was elusive, nameless, placeless, an impressionistic recital (7.2).
One of the ways Holly maintains her isolation is by only offering up generalities about herself. She appears to share some personal information, but it's too non-descript to really mean anything.
I stepped on Holly's dark glasses, they were lying on the floor, the lenses already shattered, the frames cracked in half (11.8).
This occurs after Holly finds out about Fred's death, and the smashed glasses show us that she can't maintain total isolation from the people around her. What were once the symbol of her isolation – her glasses – have been destroyed when the narrator sees how profoundly impacted Holly is by the loss of her brother.
Only: what other friends of hers did I know? Perhaps she'd been right when she said she had none, not really (15.7).
The narrator thinks this after Holly's arrest when he can't get a hold of anyone to help her. He realizes that her isolation is so complete that only he and Joe are concerned about her well-being. This is one negative consequence of her isolation.
"My husband and I will positively sue anyone who attempts to connect our names with that ro-ro-rovolting and de-de-degenerate girl" (15.8).
This is Mag's reaction to hearing from the narrator after Holly's arrest. We can't really blame her for not wanting to help Holly since the woman did steal her fiancée, but this does show us the consequences of Holly's behavior. Part of her isolation stems from a lack of care with other people's feelings, and this results in anger and bitterness from those affected.
"I told you. We just met by the river one day: that's all. Independents, both of us. We never made each other any promises. We never – " she said, and her voiced collapsed […] (18.14).
After Holly turns her cat loose, she tries to convince herself that she's happy being alone, but we see that this really isn't the case.