Dark
Successfully complete the game in story mode to unlock Dark as a playable character

Easy weapon points
Set the difficulty to easy and play the game in story mode. Save the game just before ending a level and change the difficulty to hard. Complete the level to get the weapon points for the hard difficulty setting. Change the difficulty back to easy when starting the next level

Variel
Defeat Variel during game play in story mode to unlock her as a playable character