Your code starts at @ENEMY, Im guessing thats what makes him appear. You should be able to tell regardless of the colors, sorry I can only help so much.
Not entirely sure, your code is a bit confusing so I cant modify it myself. However I think something like this could work. You may need a GOSUB to check if the enemy should display everytime u enter or leave.
It is hard man, i understand your frustration. I understand the basics but it gets very complicated. Best way to learn is start coding simple programs, use the manual and you can easily code the area of a circle w/variables.