Courier 6 is an android. Old World Blues will hint at this with the scientific experiments in the lab there, and Lonesome Road will finally reveal that Ulysses is your creator.
Think about it:
- From Fallout 3 we know androids can be built that are so similar to humans that even they don't know they're androids. From this, it seems androids in the Fallout world can have fully functioning metabolisms, blood, skin, and other systems that make them very similar to humans. The fact that you can use stimpaks and food to heal does not discount the possibility that you're an android.
- Your character survives being shot in the head. This is possible for a human but unlikely. That you survive suggests unnatural toughness.
- We know Ulysses has some relationship to the Courier. It can't be familial or romantic due to the story. They could have been friends, but you don't seem to know anything about the guy. Him being your creator makes sense.
- We so far have zero information about Courier 6's past. In fact, it also seems like Courier 6 may not even have any memories of his/her past. This would make sense if you're a fairly young android.
- Lonesome Road is supposed to tell the story of you and Ulysses, but that story needs to make sense for any character you could have possibly created. One of the only stories that could make sense is that Ulysses built you. Any other stories about your past might conflict in some way with the character you've decided to play.
- The Cyborg perk did not return from Fallout 3. Could this be because you're already fully synthetic?
- Old World Blues has a perk/trait available that can respecialize your character. To explain this, it may be a reprogramming.
Now add your own wild speculation!