All characters come from somewhere, and you should know where that is, even if it is unlikely that an RCM will ever visit that time and place again during the game.
For starters, a character should come from a time and a place, be it actual history, expanded history or the future. Also, you will want to know the back-story behind the character's Focus and Powers (even if the character isn't certain). This collection of facts is summarized as the Origin.
It is further helpful, but not mandatory to know:
You can fill pages and pages with background material for even the dullest of human beings, but you don't need to. If you can answer the questions above, you will know most of what you need to know about what this character might do and why.
It is safe to assume that any character has a working knowledge of his native language, customs and relevant geography. Further, we can safely assume they are proficient at least to their core dice with any skill someone of their background would normally be proficient with. For example, modern US teenagers could be expected to know how to read, operate a vehicle (or at least ride a bike), operate a smart phone, and search for porn on the internet.