I have a character I want his identity to be a mystery throughout the story, even after he takes on the role of someone else after being mistake for that person. So my question is, who should I go about identifying him until he is mistaken, if I go with writing it in third person or just go with first person….?
I would let your POV character determine what makes the most sense. How does he reveal himself to your POV character? What words would that character use to describe him? Who does the character think this mystery character is? Don't make the mistake of trying not to confuse the reader. Have the POV character's subjective view of the situation warp the narrative. Keep the reader guessing. Then once the big reveal comes, it's a surprise for both the character and the reader--and that's the whole point. Keep the reader invested in the character--limit the reader's knowledge to only what the character knows. If the character only identifies this person as a collection of adjectives or a false name, so be it.
I want to write it in third (narrative) and have the mystery woven in and out the story. I'm just not happy calling him 'the stranger' or 'man in black' until he slides into the other identity
When Terry Goodkind did this, he just used 'he' and dwelled on the person's actions and thoughts. It worked for me.
To kickstart my own thought processes on these sorts of matters, I tend to turn to other fiction I know. In this case, the use of Strider versus Aragorn in The Lord of the Rings could be a good example. Skye or Daisy in Marvel's Agents of S.H.I.E.L.D. is another. Dumbledore and Albus from Harry Potter might work as one too. After examining a few of these cases, it seems to be that it comes down to 1) what the character thinks they know about the other and 2) what they believe is the most prudent moniker to assign them based on this. If you're intent on dissecting each element and aspect of this, you'll encounter such considerations as formality, social context, dialect, culture, tradition, playfulness, creativity, coincidence, dislike, distrust, assumption and good faith. This may or may not be fascinating to you, but in the end I think it's simpler just to try to put yourself in your character's shoes and go with your gut regardless. Just take not of how you would address and refer to your father, employer, nemesis, neighbor or a stranger on the street. If the situation is anything like something you've come across like that, simply copy what you would have done. If it's some bizarre situation of strange aliens or just extraordinary peril, you get to invent your own names, nicknames, titles and terminology. Yay!
It sounds as if you are trying to decide what to refer to the character by instead of a name. I've introduced characters into my current book that my MC does not know who they are by name. Until she finds out their names she refers to the villain by the car he drives (Mr Mercedes) and the femme fatale by what she looks like (Blondie). Much later in the book she comes to know their names (Daxter and Lidiya) at which point I change to referring to them by name.
