Birth by Sleep Ven to Sora Theory

The "Ven's heart into Sora" was an example.
There was an argument awhile back because someone had a theory that was completely different from someone else's but that someone else claimed they stole the theory because both of theirs were based around the same idea that he "came" up. Think of it as someone saying Roxas is a part of Sora before KH2 came out. Someone says that and then makes a theory that Roxas is Sora's clone. Someone says Roxas is Sora's Keyblade manifested into a person. The clone theorist claims the Keyblade theorist stole his theory because they're both based on the speculation that Roxas was a part of Sora, which he supposedly thought of first. People think alike and it's really common for more than many people to get the same broad idea, but different specifics or details.

 

See thats the thing, just take a look at Xehanort and Xemnas, Aeleus and Lexeaus or Even and Vexen. They barely look and act any differently from their time as complete people, to being Nobodies.
Sora and Roxas... for one the hair is coloured and styled different, more so than any other Nobody. Two, their attitudes are contrasting alot of the time, even though they do seem to have happy-go-lucky natures at time, Roxas is a lot more moody and angrier than his Somebody, attacking both DiZ and his computer out of rage. DiZ even says that Roxas should give some of that hate to Sora.
I mean, for a Nobody, Roxas is special. Apperance or otherwise, he's more like his own person than just a copy of Sora.

That is why I believe that it is the Ven 'side' of Sora that comes out into Roxas.

 

And that would also explain why people (Axel in days and KH2FM+ for example) asks Roxas if he doesn't have a heart after all.
He probably has Ven's heart, or some kind of memory/shadow of it.