irvanovich says:
If it were me, I'd create one level 20 sheet with notes as to how this character behaves (pulls punches, "misses" on purpose, falls "unconscious" with +20 Bluff to play dead, etc.)
Yeah, since he is level 20 after all, you should make a level 20 character sheet, and yes, maxed out bluff should be a requirement (perhaps with Skill Focus (Bluff) just to make sure). I'd also suggest to keep notes as to what abilities the character should not use in order to avoid blowing his cover (or which abilities to used in a reduced state). He'd also have to be careful as to what magic item he carries around, because a quick detect magic on your players' part will quickly show that something is up...