Unlock Boss Rush
Beat the game with best ending (All bosses)

Unlock Julius Mode
Finish the game with the bad ending

Unlock New Game +
Beat the game with best ending (All bosses)

Unlock Sound Test Mode
Complete the game with the best ending and the Sound Test Mode will appear on the Main Menu.

Rare ring at start
Have Aria of Sorrow in the GBA slot when creating a new game. You will start with the rare ring item in your inventory.

Hell Flame Soul
Complete Hard Mode with Soma and you will get the Hell Flame Soul.(

Play as Alucard In Boss Rush
After beating the game with Soma getting 100% completion, (thus unlocking boss rush mode) AND beating the Julius mode, put in the Konami Code at the Konami logo. Wait at the title screen until the logo shows up again, and put in Up, Down, and then hold up until you hear a door opening sound. Then enter Boss Rush mode. You will be Alucard with his Alucard Set.