Protagonist / N.O. Channel
A lot of that performance comes from his older brother Tasuku. Tasuku was the baseball star who left for America, the person Mamimi loved, and the standard Naota quietly measures himself against. Naota even carries Tasuku's bat while refusing to swing it. Not trying lets him avoid discovering whether he can live up to the person he has built his brother into.
Then Haruko Haruhara arrives and makes detachment impossible. She opens an N.O. channel through Naota, uses him as a passage for machines and weapons, moves into his house, and becomes another person whose attention he wants even when he knows he should not trust her.
Naota's growth is not that he suddenly becomes mature. It is that he gradually stops using indifference as protection. By the end he can swing the bat, admit what he feels, confront Haruko, and let both her and Atomsk go without pretending none of it mattered.