When you change player character they get the characteristics you set them them to have when you created the characters. They also each have their own inventory. If you are experiencing something else, please say exactly what it is that it's not working, and eventually show the code you're using.
As for the second question about GUIs you don't say how you have set the pictures in the GUI, are they buttons? If so, you just use button.NormalGraphic.
As for the second question about GUIs you don't say how you have set the pictures in the GUI, are they buttons? If so, you just use button.NormalGraphic.