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One idea might be to think of his motivation and judgement. How does he know how to comfort her? Or that comforting her is the right thing in this situation? He must have some source of informatio...
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Source: https://writers.stackexchange.com/a/32268 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
One idea might be to think of his motivation and judgement. How does he know how to comfort her? Or that comforting her is the right thing in this situation? He must have some source of information of what would be the right thing to do. Normally I'd assume that comes from the social environment he grew up in (e.g. his mother comforting him when he had nightmares as child), but you wrote he's grown up in total isolation. So the only way he could have come to this information is that he _himself_ experienced before (or quite possibly still experiences from time to time) what the girl experienced now, and he knew what he _would have wished_ to get. That is, portrait him explicitly as _weak,_ but it is _exactly this weakness_ that allows him to understand her and thus help her in her weakness, and in turn helping her makes himself stronger. Another option is to consider whether the person with the nightmare needs to be a girl. Of course I don't know your story, so I can't tell, but maybe your story can work just as well if he comforts a boy who had nightmares? Or if the girls plays an important role later, maybe split it into two experiences, the first one with a boy, and then a repetition of the same situation with the girl (the protagonist's experience that he can do it _again_ could also be a very helpful one for his development).