Apparently, once you beat the game and complete the Battle Memo which records all enemies' poses (that also includes their backs), you unlock a little secret to make the game harder in your second playthrough. Yes, you need to have all the poses of the enemies, and you MUST find all rare enemies at least once to unlock, including hidden bosses. Are you up to unlock the Hard Mode doing all that? I'm going to work hard just for it. :)
[spoiler]If you really do NOT want to do all that to unlock it, just type "HARD MODE" as your name in the Sanctuary of the first chapter to unlock the mode instantly. Yeah, the ending secret just tells you how to open it yourself, not the game opening it for you.[/spoiler]
I believe this is old news. However, I have yet to actually try it.
Mother 3 was hard on it's own...