Well, after I couldn't get it unlocked on my game I checked ffinsider.net. I read that In order to unlock Nemesis, you need to capture 10 of every monster from every area, and fight and defeat every monster from Original Conquest at least once.
So...if anyone knows of anything else that has to be done to unlock Nemesis, feel free to say.
- Capture 10 monsters from every area check
- Fight and defeat every monster from Original at least once check