pming
Legend
Hiya!
Now, I'm just spit-balling here, and this suggestion is pretty far out there, but, uh, maybe talk to your DM?
Really, just ask him/her and explain your character concept. There is absolutely nothing stopping you and the DM from coming up with some cool back story to explain this. Maybe your character's mother was under a curse, or maybe you were accidentally baptized in diluted waters form the River Styx, or any number of other fantastic ways. Then all you and your DM have to come up with is the mechanics; maybe you get 2hp per level. Period. But you are immune to critical hits as nothing can "pierce your skin"; broken bones, bruised muscles, traumatized internal organs...yes, but you never "bleed" from this and your skin is never 'pierced' (re: you can't take critical damage...you can be criticaled for other effects, but you just don't take extra HP damage).
The bottom line is that for things that lay outside what the guidelines/rules in the game give you, you make


up. That is the nature of the game. Imagination and creativity. Do that. 
^_^
Paul L. Ming
Now, I'm just spit-balling here, and this suggestion is pretty far out there, but, uh, maybe talk to your DM?
Really, just ask him/her and explain your character concept. There is absolutely nothing stopping you and the DM from coming up with some cool back story to explain this. Maybe your character's mother was under a curse, or maybe you were accidentally baptized in diluted waters form the River Styx, or any number of other fantastic ways. Then all you and your DM have to come up with is the mechanics; maybe you get 2hp per level. Period. But you are immune to critical hits as nothing can "pierce your skin"; broken bones, bruised muscles, traumatized internal organs...yes, but you never "bleed" from this and your skin is never 'pierced' (re: you can't take critical damage...you can be criticaled for other effects, but you just don't take extra HP damage).
The bottom line is that for things that lay outside what the guidelines/rules in the game give you, you make





^_^
Paul L. Ming