This is how I see it. Roxas was supposed to be Sora's nobody, which he is but because Sora's being was restored to him and Roxas had nothing to go on since Sora was getting it all back, he had to find another source and that was Ven which was also in Sora's body. So basically I'm saying that when Sora released Kairi, what's left of Ven's, and himself, Kairi got hers back, Sora's heart was restored from being a heartless, so Roxas had nothing to go by and since Roxas had nothing to go by he took after Ven. Like how Xehanort's Heartless commandeered Riku's body in KH1 and booted Riku out. so some part of Ven is making Sora's body appear like Ven which is Roxas. I'm not saying all of Ven's fractured heart is in there but I saying he had to go by something that was available.
