A Monster Calls
How we cite our quotes:
When Conor started having that nightmare, that's when Harry noticed him, like a secret mark had been placed on him that only Harry could see. (3.8)
Harry and Lily are the two people who see Conor, but in very different ways. Harry makes Conor his enemy, while Lily's view of Conor is protective, even if she does have a big mouth.
And so he waited by himself, leaning against a stone wall away from the other kids as they squealed and laughed and looked at their phones as if nothing in the world was wrong, as if nothing in the whole entire universe could ever happen to them. (18.4)
His classmates are preoccupied with their phones; Conor's preoccupied with death. He definitely got the short end of the stick when cancer chose him out of all his friends.
Conor hadn't heard a word of his lessons in school, but the teachers hadn't told him off for his inattentiveness, skipping over him when they asked questions to the class. (18.33)
When Conor beats Harry up and yells, "Never invisible again," he's not just talking to Harry. He's also using Harry as a punching bag for all his frustration with how everyone is treating him. It's like a mini manifesto.