At 00:30 seconds into the trailer you see Credence Barebones seemingly whisper the word "Nagini" through a circus cage to the young woman dressed in a blue scaly outfit, at which point she literally turns into a snake. This woman is repeatedly shown in the movie's promotional material including several that indicate a close relationship with Credence, such as this one:
Now, to give you a quick summary of what is happening with this woman (played by Claudia Kim) and her character, let me explain:
