[Adobe Illustrator] Create a tool for merging shapes with floating lines

I feel like this is a basic question, but maybe I'm using the wrong terms to search. But how do I make the circle and the diagonal line the same object, so that I can fill the circle without the line becoming hidden-- or be able to change the stoke weight, color, etc. I tried the shape builder tool, compound path and Unite under pathfinder options but none gave me what I am looking for. I have a lot more complex shapes to make (letters with "floating" or non-intersecting lines) so this problem is just going to compound over time.