This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Remember the Tower of Walz, which sank to the bottom of the ocean after
you had grabbed some of the crystal shards? Well, do you remember that
there was one shard that you couldn't reach?
You can get this crystal shard in a sub-quest later in the game. Once
all eight crystals are destroyed and you're in the game's third world,
get the submarine. Pilot over to where the Tower of Walz used to be, and
Your time here's limited, so act quickly. Go back to the room where the
crystal was. You'll now fight Gogo for control over the extra shard of
the crystal. Remember: To beat Gogo, do as he does...
If you won, you'll get the game's secret job: Mimic. Get it and get out.