So I have a static terrain and a character on top of it, I use mixed lighting and everything works properly when I build the game and run it, but when I am in the editor it's all just dark. The static terrain is completely dark. I'm sure this is a newbie problem, but what am I missing?
When the editor is still busy baking your lightmap data, it will appear dark. The reason why this is not an issue for your build is that the build ignores the incomplete backed data.
You will probably see something like "Baking... [ETA xxx]" on your Unity Editor (bottom right-hand corner) which indicates that the baking is still busy.
Alternatively, you can clear the baking data or manually start the baking.
Also, check that under Lightmapping Settings, Bounces > 0. With 0 bounces the shadows are completely black.