A downloadable D.D. EX for Windows, macOS, and Android.
It's an amazing game, very high quality!
A bit overcomplicated though and really hard to achive certain endings without the guide. I think, the best ending is actually the "Killer Ending 1" as it explains everything and is a happy ending. I guess the game is made to be replayable. I wish though that there was some sort of fast forward button for seen dialogues as they do in the most over VN games.
