This is one of the worst n00b questions ever...
The BFU, who is most likely Riku, was in the teaser at the end of Kingdom Hearts and Kingdom Hearts: Final Mix. Those teasers are Another Side, Another Story and Deep Dive. Nomura confirmed that one of the Unknowns in Another Side, Another Story was Riku. There were only 2 Unknowns so it's obvious which one he was. The other Unknown was the DWU, an early version of what became the BHK. In Deep Dive, there are 3 new Unknowns: the GEU, SU, and Mickey. The Glowing Eye Unknown (GEU) is either a Heartless or a person with the powers of a heartless or is just using them. Either way it doesn't matter because there is no proof that he still exist. The Sitting Unknown (SU) is thought to be the Enigmatic(sp?) Man (EM). Why? Because s/he says the same thing to the GEU that the EM in Final Mix said to Sora, "He looks just like you." Mickey is obvious.
Final Mix is the Final version of Kingdom Hearts. The English version of Kingdom Hearts got 3 new optional bosses. This pissed off the Japanese owners of KH. They got all bitchy and Nomura had to do something so Final Mix was made. In Final Mix they got new cut scenes, those 3 optional bosses plus the EM, 2 new keyblades, and Deep Dive.
There that's everything you need to know.