Carl's speech to Donut about masks, which Rosetta watched and wept over for its hopefulness.
Rosetta watched Carl's speech to Princess Donut about masks, and she wept. She wept at how hopeful it was, and she wept at how cynical it made her feel in the moment — she pitied both of them, thinking how naive it was that Carl believed Donut would ever be able to remove that mask.
As she stands on the edge of what she expects to be her death, she reconsiders: was she wrong? Maybe so. The speech, set against her own creed that everyone who enters the dungeon dies in the dungeon, becomes part of what pushes her toward fighting for her friends rather than lying down.
The moment of Carl's hopefulness that quietly cracks Rosetta's cynicism.