"Nerissa" HAS to be Faith. I can prove this through the knowledge of multiple playthroughs. Be patient and read all of this to fully understand.
Before I get into it, we all know that Telltale does EVERYTHING for a reason, and even the most subtle of hints have been carefully placed in the game by them,
some examples are: Georgie's "stop laughing" line and the fact that his choice in cigarettes matches the killer's unpopular brand. Clues like these are present well before act 5. On that topic, there are actually a load of other subtle clues, some that are the more relevant to this topic are as follows:
while being interrogated in Act 2, the Woodsman mentions that faith stole something and in act 4 at the Tweedle's office, a document mentions that what Faith stole was a photograph from the crooked man.
These are some things hide in plain sight and Telltale do these things intentionally!
if you've never played a Telltale game and are unaware of how subtle they are, you should also consider all the research they would have to perform before writing a story about fable characters; they CANNOT create an event that would conflict with fable comics, or the history of the characters.
SO HOW CAN I PROVE NERISSA IS ACTUALLY FAITH?
Keep in mind, all of Telltale's research and of course, their notorious sneaky nature, these events are NOT coincidences:
Now, consider Faith's original story that was intentionally drilled into our heads back in act 1, "She flees her town while in a disguise". Its not a coincidence that "Nerissa" does this at the end, nor is it a coincidence that Bufkin quotes that very line during Bigby's thoughts at the end.
The lines Bigby remembers are mostly similar dialogue between the two girls, there's no other reason to throw in Bufkin's line other than to differentiate which one is alive, clearly implying that Faith disguised herself and left. Think about it, when Telltale was choosing which lines Bigby recalls, that line wouldn't be there if they weren't trying to make it clear who was who.
The thought of Nerissa being alive seems more logical at first, but think about Faith's personality in contrast to Nerissa's, Faith is clever, witty, and can be deceitful at times, Nerissa is more emotional and not quick on her feet like Faith. So HOW ON EARTH could "Nerissa" be so prepared to lie to the entire community INSTANTLY after her spell was broken? Furthermore, the next day at the crack of dawn she is already packed and ready to leave town, surely this decision had to have been planned.
This begs the question, If it was truly a traumatized Nerissa confessing herself to Bigby at the end why would she avoid telling Bigby she glamoured herself as Faith in the beginning? We know one of them is the other, so a humbled, emotional Nerissa would surely have opened up about this...UNLESS (take a guess) she is actually Faith trying to escape!
The very last line in the entire game is a quote from Faith where she says, "See you around, Wolf."
To me, this is a HUGE indication that Faith planned on seeing Bigby again. Surely the decision to make it the final line in the game must've had Telltale feeling that they're making it obvious.
(This is certainly her voice as the subtitles for this line are blue.)
Additionally, something that irked me during the actual end conversation with "Nerissa", is that she seems to answer some things with ambiguity, and it almost seems as if shes trying to distance herself from all the conflict, "Faith did this, then Faith did that" she tries to blame it all on the "Faith" that Bigby believes is out of the picture. This is more clever writing from Telltale and closes the case in my eyes, but on the topic of how this could all be possible, it is noteworthy that the butcher's shop in Act 4 has a chalkboard which shows that Faith, Lily AND the real Nerissa purchased glamours, so this would support the idea of the real Nerissa being killed in Faith's place.
Let me know what you think, I have yet to find anything that conflicts this theory overall, I think i've cracked it.