Same as we would any other history! We totally understand minor characters with not enough background, so don't worry about that too much. (If you have speculation on her background that you would like to include in her canon history section, that's also allowed, by the way.) Such as it is, the same rules apply: what you do have for canonical history should influence her City history as much as anyone else's — which is to say, not much.

More than anything, we would probably say that this is where a strong personality section would help us immensely in the absence of more concrete history, just to make sure we're all on the same page. Does that seem fair?