BHK is his own person....Where he lives. I think that in all of the different worlds, everyone is there own person....But when it comes down to it, they aren't.
Example-
Sora = Present World.
When he was split in Hollow Bastion, a dusk was born in Twilight Town while in the Present World, a normal weak Shadow Heartless was created. When Kairi restored him though while he was still changing and such, I guess the Dusk had transformed as well in the same manner.
BHK = Twilight.
Since he was basically created the way he was, he and Sora would need to be combined in order for them both to actually be powerful, weild the Keyblade at full strength, etc. Since of course, he was created in his own home-world, he had his own heart and could weild a Keyblade but when it boils down to it, Sora needs him and he needs Sora.
That was just an example to sort of show what I think. Of course, we know the event at Hollow Bastion makes no difference but this was just to show that BHK could possibly be his own person, own character but both are a neccessity for an all complete being.
(Note. That was JUST AN EXAMPLE. AN EXAMPLE. AN EXAMPLE. I know that it isn't possible now but I was using it to show what I think. And I'm certain BHK has no real relatives that are an impact in the game what-so-ever.)