The game where those kids are throwing snowballs on the first fight, is FFT Advance, the GBA one.
This FFT is the PSX one.

And, to answer your question....

To get the Dark Knight you must accomplish the following requirements with the unit you want to become the Dark Knight:

- Master the Knight and Black Mage Classes. You must learn all the abilities with both of them.

- Reach Lv 8 with the following classes: Dragoon, Geomancer, Samurai, Ninja.

- You MUST kill (The enemy defeated must turn on a crystal or treasure chest.) at least 20 enemies with the characters that is going to be your dark knight.


Good luck getting one. =)