Destai pretty much summed up my opinion on the relationship between Sora and the BHK. Though I don't have much of a clue what the other side of the heart exactly is, I'm confident that the BHK is the other side of Sora's heart just as Sora is the other side of his.

As far as the Organization goes, I'm not so sure about what I think of them. So far they're the most mysterious part of Kingdom Hearts II. There's a whole bunch of them that haven't been identified yet. #12 is the highest number we're aware of. Vexen was 4, Lexaeus was 5, Zexion was 6, Axel was 8, Marluxia was 11, and Larxene was 12. Assuming DiZ is 1 that leaves 2, 3, 7, 9 and 10 unmasked which only leaves more questions. So there's at least five that we don't know about yet. If 1 turns out to be someone other than DiZ that'd put us at six. Of course, there *could* be more. However, if I'm not mistaken, throughout Chain of Memories both Marluxia and Larxene are regarded as newbies, perhaps recent additions to the Organization. If that is true, it doesn't look like there's going to be a bunch of members with numbers above 12.

Also, we've seen Riku, Mickey and the BHK don the cloaks that are the trademark of the Organization. That might mean that they have joined the Organization, which I believe Riku and Mickey, at least, have. Or for some reason they're as many suggest "under-cover." Seeing as Riku meets with DiZ at the crossroads at the end of Reverse/Rebirth, I get the feeling that DiZ would know why he and Mickey wear the cloaks, so I just get the feeling that the under-cover business is bunk.

There's also the "Enigmatic Man" from Final Mix (and perhaps the Unknown that led Sora to Castle Oblivion?) He could be Unknown #2, 3, 7, 9 or 10 or he could be another. I believe it was Vexen who said he knew "an" Ansem, but not the Ansem that Riku knew. I get the odd feeling that this Enigmatic Man is the body which the Ansem we know cast off. He seems fairly important to me, so I wouldn't be surprised if he turns out to be DiZ's right-hand man.

As far as the Organization's affiliation with the darkness or light, I'm not quite so sure. Though they suggested that they are neither, they seem to use the darkness more than the light and DiZ made it clear that nobody has been able to manage a balance between light and darkness, the twilight. The only inkling to their motives is the Unknown who manipulates the Beast by holding the magical rose that the Beast is bound to. He uses the Beast against Sora, as it seems, so that would suggest that they want to stop Sora, but I don't think we have any knowledge of why they would do that.