I am wondering about this for a while now. In Kingdom Hearts 1, Sora uses Riku's key to release the hearts within him - namely Kairi, the hearts of the princesses and the sleeping heart of Ventus. In the process his body fades and he becomes a heartless, additionally forming Roxas. What I am wondering about is what exactly happened to Sora's body. The series teaches that a body with a strong will devoid of a heart becomes a nobody because the nobody is formed from the remaining will and possesses the body. The heart becomes a heartless or takes shelter somewhere until it can return to the body (as with Ven). Does this mean that Roxas didn't only take a part of Sora's memories, but also his body - which would mean that Sora is just a walking heart up to the point he merges with Roxas in KH2. Taking Ansem SoD in consideration it is very well possible to survive for quite a time without a body and just a heart (even with a very dark one) since Ansem SoD was able to survive for about 10 years in that state. This would give an additional explanation why Sora needed Roxas so much - he needed his body back before he would degenerate.
The other possibility is that Kairi restored Sora's body from scratch. This would mean that we have two copies of Sora's body - the restored one and the one Roxas possesses and whose shape is altered due to Ven's sleeping heart that took shelter in it. When they fused, one of the bodies dissolved because only one was needed.
There might be another possibility I can't think of, so I'd like to hear your opinion. If you find mistakes in my assumptions, feel free to point them out. I tried to recreate the stuff as far as I understood it.
The other possibility is that Kairi restored Sora's body from scratch. This would mean that we have two copies of Sora's body - the restored one and the one Roxas possesses and whose shape is altered due to Ven's sleeping heart that took shelter in it. When they fused, one of the bodies dissolved because only one was needed.
There might be another possibility I can't think of, so I'd like to hear your opinion. If you find mistakes in my assumptions, feel free to point them out. I tried to recreate the stuff as far as I understood it.