Right-Click the Illusion installer and use Locale Emulator then choose Run in Japanese
It will show you where to install the game (You need to make a new destination! If you use the original destination, the game will not start and will show an error.)
Keep clicking (N) until the game is installed to the new destination.
Try to run the game by Right-Clicking the game and use Locale Emulator then choose Run in Japanese, and if it works properly then you can proceed to install the Real Girlfriend HF Patch.exe