A Deconstruction of the Faith vs Nerissa Theories
Bigby experiences a set of flashbacks at the end of the game that heavily imply that Nerissa and Faith were in fact the same person. Various parallels exist between their dialogues and interactions with Bigby, and it heavily suggested that one of them was in fact glamoured to look like the other.
What we know about Glamours
* Glamour tubes are sold to Fables who do not look human or who cannot assume a humanoid appearance so that they can avoid detection in their new home. They are made by the witches on the Thirteenth Floor of the Woodlands, and because they are quite expensive many lesser-off Fables use black market glamours made by witches such as Aunty Greenleaf.
* A glamour is made using the DNA of the person the user wishes to resemble. This can be seen when Bigby opens Lily's glamour tube and finds Snow's hair inside.
* The effects of the glamour will wear off when the tube is opened. This happens at the Witching Well when Bigby uncovers Lily's tube and later at Aunty Greenleaf's house.
* There are no indications as to whether or not a glamour has a time limit or will wear off naturally.
* The girls at the Pudding 'n Pie often used glamours to cover each other's shifts.
With that out of the way, let's move on to the supporting evidence for the two main theories:
Theory 1: Faith glamoured herself to look like Nerissa
The first theory proposes that the Nerissa we meet in the game was, in fact, a glamoured Faith. This would mean that Faith is actually alive and it would also imply that the severed head that Snow finds at the beginning of the game most likely belonged to Nerissa.
Evidence
* When Bigby asks the Magic Mirror to see Faith, it is unable to show her to him because of the purple ribbon spell. This implies that Faith is alive because it is highly unlikely that the spell would continue to be in effect after her death.
* One of Bigby's flashbacks at the end of the game is Buffkin's recounting of the story of the Donkeyskin girl who masked her appearance to flee her kingdom. I do not believe this quote would have been included if it was in fact Nerissa who glamoured herself to look like Faith. This implies that Faith faked her own death and is using her Nerissa glamour to flee Fabletown, just as she fled her kingdom in the Homelands.
* Another flashback included in Bigby's "realisation" montage is that of Dr Swineheart saying he needed to "run more tests" on Faith's head. This indicates that perhaps is was under some sort of enchantment. Again, I believe that the devs included this quote deliberately.
* The comic book adaptation of the game confirms that Faith is alive.
Theory 2: Nerissa glamoured herself to look like Faith
The second theory proposes that the girl that Bigby meets in Episode 1 was in fact Nerissa, and that Faith had most likely died before the events of the series. This implies that Nerissa glamoured herself to look like Faith so that Bigby would recognise Faith's severed head when it was later left on the doorstep of the Woodlands.
Evidence
* During Bigby's altercation with the Woodsman at the beginning of the game, the Woodsman explains that he is angry because Faith does not recognise him. While this may have been the nonsensical ramblings of a drunk, it is also possible that the reason Faith doesn't recognise him is because she is in fact a glamoured Nerissa.
* Bigby describes Faith as "sweet". However, the Tweedles seem to vehemently disagree with him and the photo that can be found in the Crooked Man's warehouse at the end of the game suggests that Faith was violent towards Nerissa. This evidence is unfortunately not very strong, however, because it can be explained away by the fact that a person's behaviour depends on their situation and who they are with.
* At the Pudding 'n Pie, Faith's jewelry box has been busted open. However, this too can simply be explained by the fact that Faith would have most likely faked evidence as part of her plan.
While there is evidence supporting both theories, I personally lean more towards the first one because I believe that it is the direction which the devs point the player in and because the evidence for the second theory isn't very substantial. What do you think? Is there any evidence I missed?