What’s the legality of making a character editor?

I want to develop a character editor for role-playing games. At first just for one game, but I want to expand it later to cover multiple games.

What laws (like copyright and intellectual-property) do I have to consider, and what is allowed in it and what isn’t? What are my limits without a contract with the game’s publisher? Is there a difference between free, commercial, and open-source character editing software software?

I am aiming at Shadowrun 5 first.