Out of memory, the dragon is an optional boss... The ending boss happens when u return to the cathedral where the game began(same place u meet the spirit girl after complete most missions and level up the heroine enough). Basically a choice there defines the end and also, the final boss or something... the Dragon is totally OPTIONAL. Keep in mind that the values of morality, lewdness, etc also affect the ending or some shit like that. Out of memory, if u don't meet the criteria or the min value for stuff or pass the required amount, the option to choose doesn't even appear... That's also the reason why the game has a "Debug Menu" in it's options to free all cg's if want/modify alignment values and money... I'm not sure, but the game has a certain number of missions, if u complete half of those or something like that, the ending is already avaliable to be watched... u don't need to go completionist mode on this if want, basically if u're strong enough and completed enough shit, return to where everything begun to check the ending choices/possibilities(Reason i don't know the conditions is... i played this game when it was released in jpn, so, not sure of anything hahaha).
same with me i tried everything even switched my language to japanese instead of just having it installed
for me was some missing file in windows folder i downloaded program Advanced System Repair Pro and fixed missing files problem
you can get at Faerie Forest dungeon where Egg Vibrator item get. add Devil Boots location
