Unity 3d C# Draw Circle and Detection

I want to draw a flat circle and when a character controller steps inside that circle I want to create a debug.log saying "you are in the circle" while the character controller is inside the circle. So first thing is I would need to do is draw the circle and then give it a way to detect the character controller. However I'm not sure how to do either. I think for the circle I would need to draw a flat circular collider but I'm not positive. Do you guys have any suggestions on how to do this?