I cleared Hard Mode with the middle character. It was an excellent game. The sound of the crystals shattering is incredibly satisfying, and it makes defeating enemies really fun. The description mentions two levels, but since Hard Mode offers a completely different gameplay experience, it felt like I got to enjoy the equivalent of four levels. The boss attack patterns are also unique and really cool.
That said, I never quite figured out what "Core Stability" actually means. Also, in Hard Mode, the player ship change from the normal, which was a bit puzzling. But that might be intentional and not a bug.