First, reread the story carefully, underlining or making notes of everything about Roger that give you ideas about what sort of person he is. For example, when we first meet him, he tries to snatch the purse of a woman all alone on a city street at eleven o'clock at night. This act suggests desperation, but it doesn't instantly prove he's a bad person; after all, he may be homeless and starving and finds that he has no choice but to get money or food any way possible--we don't know yet. (Notice that Mrs. Jones doesn't assume he's bad, either.)
He knows enough to say "Yes'm" to her instead of "Yeah" or worse, which suggests that he isn't yet cold, cynical and hard. He probably doesn't live on the streets and run in a gang.
Keep gathering hints like this until you've found all you can, then check your notes and decide what this tells you about Roger as a young man. Is he simply an example of misguided youth? Is he misunderstood? Does he simply lack adult supervision and respect? When you decide, you state your understanding of the sort of person he is as your thesis statement, then use all of your examples from the story to argue for that characterization.