"Yeah," she said in response, trying to play it cool. "Who'd want someone like that, anyway?"
He recognized that tone, that waver. She'd become too human in his time there; she'd become a bad liar. Or maybe it was just now.
Regardless, his instant realization took him so by surprise that he sat up and turned away from the river to face her.
He forgot to hide the look on his face, though, and she read it plainly enough. She was actually dumbstruck, which was new.
"No," he said, not wanting to hurt her feelings. "No," he said, not wanting her to think that he meant what he meant. "No," he said, regretting what he'd just said, but it came out as just "no, no, no."
She didn't know how to react. Her own thoughts were unfamiliar to her. She tried to fly away, but she couldn't, so she just ran.
"No," he said, quietly, to himself, as he realized what had happened. The spirit world hadn't changed him, but he'd begun to change it.
He wandered off that night, hoping to find a way out, before he ruined everything.