TheWertle is currently translating an interview found in the Kingdom Hearts II: Final Mix+ Ultimania:

Kingdom Hearts II: Final Mix+ Ultimania Nomura Interview

The full translation will come soon, but it has been confirmed that the names of the three Knights in the secret ending are related to elements just like Sora, Kairi, and Riku are, and that two of the knights are indeed Aqua and Ven. Update: We now know the name of the Enigmatic Soldier is Terra thanks to another interview with Nomura! Click the images to enlarge them:

Sora means Sky - Ven means Wind
Sora means Sky, Ven means Wind
Nomura has also hinted that Ven and Roxas may be related in some way!

Kairi (similar to Umi) means Sea - Aqua means Water
Kairi means Sea, Aqua means Water

Riku means Land - Terra means Earth
Riku means Land, Terra means Earth
This is the Enigmatic Soldier for those who don’t know already.