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.
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.
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.
His classmates kept their distance from him too, like he was giving off a bad smell. He tried to remember if he'd talked to any of them since he arrived this morning. (18.35)
Sometimes we can become so isolated we barely notice our own loneliness; not talking can become the norm. It can cease to surprise you when you realize you haven't used your voice that day.
His grandma still hadn't spoken to him about the destruction of her sitting room. She'd barely spoken to him at all. (21.5)
If his teachers won't punish him, Conor thinks perhaps his grandma will. But she's just one more person who's more focused on his mother than on him. And hey, she's not exactly enjoying herself over there either.
And then one day the invisible man decided, the monster said, its voice ringing in Conor's ears, I will make them see me. (23.12)
With each story, the monster plants the seeds (because it's a tree—see what we did there?) of Conor's next outrageous act. Each time it appears, the monster raises the stakes for Conor's misbehavior.
There are worse things than being invisible, the monster had said, and it was right. (24.50)
One thing that might be worse than being the mom-with-cancer kid is being the mom-who-died-of-cancer kid. Conor will no longer have her to talk to after a rough day at school, which could certainly make the rough days even worse.
Never invisible again, the monster kept saying as he pummeled Harry. Never invisible again. (24.13)
Of course Conor was the one who was actually saying this, but just as with the wreckage of his grandma's living room, he needed to pretend that the monster was involved, even if it's just to himself. He's already shouldering the burden of his mom's destruction; he can't also be expected to shoulder the burden of his own.
I see you, read the fourth, with the I underlined about a hundred times. (25.24)
Just as the most hurtful thing Harry could do is pretend not to see Conor, the most loving thing Lily could do is remind him that she does. If this moment didn't make you cry, well then we don't know what will.
"What's the use of you if you can't heal her?" Conor said, pounding away. "Just stupid stories and getting me into trouble and everyone looking at me like I've got a disease—" (27.27)
Conor experiences disease vicariously through his mother. To be ignored by his peers for her disease makes him feel as if he's the one who's sick.