"Whether he wore a helmet or not, whether he was eating or sleeping, whether he was sad or happy."
"His expression never changed."
...I see.
"I read in the materials that if you want to make someone laugh, you need to act interesting. I didn't know what interesting meant, so I imitated the people in the materials."
"But Lin still didn't laugh."
A smile?
Lin suddenly realized he had forgotten when he had last smiled. No, it seemed he no longer knew how to smile.
A smile was the movement of the zygomaticus major and risorius muscles. Could he make himself smile by controlling those two muscles?
No.
He tried to move his facial muscles, but it was much like how he could not move anything below his neck. He simply could not change his expression.
"Doctor Mei can smile. Vill-V can smile. Kevin can smile. Delta can smile. Elysia can smile... Everyone can smile. They use smiles to express their feelings, and smiles to hide their feelings. Only Lin uses his helmet as his face. He has never smiled."
The helmet...
Without realizing it, the helmet had become Lin's defining symbol. There was always only that azure visor between others and Lin's face. No one had seen what he looked like beneath it. Perhaps some people guessed that his face beneath the helmet had its own joys, angers, sorrows, and pleasures—but Prometheus, who could see inside, knew...
There was nothing. The only difference between Lin's face and the visor was their material and appearance. Both were equally unchanging.
Rather than saying he wore a helmet, it was more accurate to say that he was displaying his face in another form.
This helmet was his face.
After trying, Lin asked Prometheus, "What about you? You've never smiled either."
"I can."