Here comes the inevitable problem of being a child (of whatever age, truly) that Wei Wuxian was fond of now, and particularly that he was saying goodbye to. Without Buttercup insisting on some overt emotional display, he doesn't try to make one, it not being in him to do so, but he does reach out and stroke a hand over her head if she lets him get away with it.

"They will," he says, "Because you're cunning enough to make them useful. No dying, promise? You grow up into the person you'll be whenever you're older. Make it that long."