How do you get Sora in kingdom hearts2

I'm presuming while you're playing as Roxas. You need to go to the haunted mansion, through the lab. Then battle Axel and continue through the doors. There will be a corridoor with pods at the side, where you can see Donald and Goofy. Go through the door at the end, and, voila, a clip. You then merge with Sora in the clip, and play as Sora.