Hello!!!
To get the fifth shadow star, you must do the shadow route and before the part where he covers your eyes, exactly just when you leave your room and you are like in the main room, I don't know if I understand myself. Well, you must touch the upper right corner, near where there is like a staircase. Touch near there because the place is not so exact. Doing that will take you to another room... and you will get a bad ending but that's the route.
And that it, I hope that it works for you ;3