A slight hint of confusion appears upon the boy’s blank expression, he literally just said that there are strangers willing to rob a young girl like her, and still she decides to ask a stranger for help. Sure, the boy would never steal from a child, but it wa still quite odd for her to just ask him like that. 

Regardless, it would only be decent to help a young damsel in a time of need, just like the great Arséne Lupin would do, so without saying anything, he just nods his head, and starts to accompany the young lady.