T. K. Arispe

 If you're playing the original Red, Blue, and Yellow on a regular monochrome Game Boy, the walls in Fuchsia Gym really are invisible. But if you're playing on a Game Boy Color (or an equivalent console that supports color), you can actually see them a little! I played on a GBC, so navigating the Fuchsia Gym wasn't quite as challenging as Koga may have intended.