Complete the indicated task to unlock the corresponding darts. Although the game does not list stats, the better darts give an advantage. For example, if you miss-throw beginner darts, you are likely to greatly miss, but doing so with the highest ranking darts can still allow you to directly hit your target.

Beginner Darts Rank C: Automatically available.
Intermediate Darts Rank B: Complete a match against a medium difficulty darts opponent in Club SEGA Theater Square in Kamurocho.
Expert Darts Rank A: Complete a match against a hard difficulty darts opponent in Club SEGA Theater Square in Kamurocho.
Paul Lim Special Rank S: Complete Dart Substory #13: Showdown With Paul Lim.