I unlocked the Halberd by clearing Event #3. This opened up two more cells. The two hints I unlocked are:
1. Clear the ???? Event
2. Brawl on the Halberd Stage 10 Times
I'm guessing the symbols in the boxes mean that 1 is a hidden stage, and 2 is hidden music. I'll post again when I find out what they contain.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.