Did a few tests with the Mystic Forge in Lion's Arch and noticed you can enter it without getting kicked out. You can slowly step down until your skills & utilities are locked. In this weird position, you get an x-ray view of your character. This gives a couple of hints which character-models are truly linked and where the real differences are. It was fun to try it out with different genders and races. There was however one thing that appeared to be the same with all characters, a glowing 'burning' orb right in the center of the chest.
The Heart? Maybe. But there is no orb in the head for the brain. Could be the soul/spirit, as even Sylvari seem to have this glowing orb. Any explanations or guesses?