Ok, so after birth by sleep came out there have been a lot of theories regarding the connection between Sora, Ventus, Roxas and Vanitas and I just wanted to put mine out there. (I know the game came out a while back but I am really lazy and didn't want to bother with putting my theory on the internet, I'm only really doing it now because I just saw a horribly stupid theory and I feared for humanity)
So I am going to break this up into 3 parts Sora/Ventus, Ventus/Roxas, and Sora/Vanitas.
Sora and Ventus
So first of all what connects Sora and Ventus, well as Sora is being born his heart takes a sleight detour as he notices Ven fading away into nothingness xehanort having broken vens heart (hehe see what I did there). Anyway Sora then after talking with Ven decides to connect his heart with Vens so that Ven wont die. About four years later a young Sora feels Ven in danger and he takes Vens heart inside of himself until such time that he can return it to Ven. Now there are still a few questions to be asked why did Sora pause for Ventus, and Why do Sora and Ventus look similar. Well a lot of people have hypothisised that Sora and Ven are related and I personally find this to be plausable for one it might explain why they look similar and why Sora would stop for Ven he felt a family member in trouble and had to help out. Another explenation as to why Sora stopped (possibly causing his mother pain due to extending her labor) is that he is just a nice person a fact that he has proven time and time again. Its also possible that Sora resembles Ventus is that when there hearts touched it influenced Soras corperal form. Or the fact that they look similar could be completly random (I have seen many people that look similar but are not related at all just look around on the beach)
Ventus and Roxas
In 358/2 days when a member of the orginazation looks at xion he see's Roxas but upon closer closer inspection we see that this is not exactly Roxas but someone that looks almost identical to him when birth by sleep comes out we find out that the character was actually Ventus. Now a lot of people wonder why Roxas looks like Ventus when he is Sora's nobody. Well there are lots of theories about Sora's and Ven's hearts touching but my theory is that Roxas is not only not a Nobody but that Roxas and Ventus are actually the same person!!! Now I know what some of you are thinking thats rediculous, how can they be the same person there personalities are completly different however if you stop and think about it then it isn't really quite as crazy as it seems. When Sora stabbed himself to releas kairi's heart subsequentially releassing his own heart however it might have been possible that he did not release Ven's hearts (what I want to know is how Sora did not die of high blood pressure I mean come on he had not one not two but three hearts inside of him during that entire first kingdom hearts game) and when Sora's body formed a nobody it still had ventus's heart inside it essentially recreating Ven using Sora's body. Roxas does not have Ven's memories because Ven's body is still alive in castle oblivian and so his memories could still be in the body because Sora didn't know anything of Ven or his past either. Roxas also didn't remember his life as Sora because Sora was changed be from a heartless (on a side note I think this was also why Sora became so weak after Kingdom hearts 1 because his body was split between him and roxas so he lost a lot of his strength and abilities and then while he was taking a year long nap that was a whole year that xion was sucking his memories and abilities.) If you look at there personalities you can notice that there really not that different point one the Ventus you know lived his entire life without any darkness or negativity in his heart all of it being in Vanitus point two if you look at Roxas at the beginning of kingdom hearts 2 or the beginning of 358/2 days this is him without any of the evil orginization baggage and he is actually a happy, helpfull upbeat kind of guy. A lot like Ven. Also note that the Ventus heart in Roxas would be whole having both light and dark in it becaus Ven had merged with Vanitus when he entered Sora. So Roxas would be able to feel those darker emotions. Also Roxas was the only nobody with the ability to feel thats because he has Ven's heart inside of him and is not a real nobody but Ven reincarnated without any of his memories.
Sora and Vanitas
Now Vanitas is an interesting phenominom as he is not a Nobody but rather a phisical manifestation of the dark side of Ventus I liken it to the pair Link and Dark Link. Link is the hero of Hyrule and is of a pure and noble heart which is why he can use the master sword the reall question is why does Vanitas look like Sora. Some people say that Vanitas doesn't look like Sora bur rather the other way around and that Sora looks like Vanitas because of Sora and Ventus touching hearts and that influencing how Sora looks. But actually in the beginning when he was first created Vanitas had no face to speak of it was only after Ven and Sora touched hearts that Van got his features and he looks like and older Sora simply because he is older he was created when Ven was a teenager so he would have also been a teenager.
-Note: Nomura has confirmed this Theory
Thats all for my theories tell me what you think about it, what you agree with what you disagree with but please no bashing people for their views or theories