I always took the phrase “She is someone’s [whatever]” not to suggest that the recipient isn’t thinking of them as a person, but that they are thinking of them as a stranger. As in, “How would you like it if you knew someone was treating your [person you care about] like that?”. It’s still a criticism for the recipient, but it doesn’t go as far to accuse them of dehumanizing anyone. Instead, it suggests you should treat them like you would someone you are close to and care about more deeply.
I felt the same way (spoilers for whoever hasn’t read it). The protagonist just kept encountering significant people where it seems like there’s going to be a struggle to overcome, leading to character development and newfound maturity, but no. He just moves on to another scene instead and they’re not seen again. It was just annoying.
The teacher that feels he’s not living up to his potential? The private school friends that he hangs out with but often finds frustrating? The childhood friend who he shares unexplored romantic tension with? The nuns whose meals he pays for despite having dwindling funds? The prostitute he just wants to have a conversation with? Her pimp, who attacks him? The potentially rapist family friend? For pretty much all of them a relevant conflict is initiated just for him to leave it unresolved, probably after labeling them a phony.
The only exception is his sister, who he sees like two or three times. And then the final conflict at the end is like: “Hey sorry for taking your birthday money so I could keep wandering around these past couple of days instead of talking to our rich parents.” “That’s ok, I forgive you. You’re my brother and I love you. But I worry about you sometimes.” “Yeah anyway, I’m bitter about the world so I kinda want to disappear into the wilderness.” “Please don’t do that.” “Ok I won’t.”