Hi! Nothing is locked under a spinoff. They are all optional. You need to have a certain affinity with the characters to unlock it. Its 100% possible without paying anything.
In the game when I chose the right choices it said my affinity raised up (i reached around 80%) and then after that i checked the "characters" screen (it showed 69%) and then it was like it never gone up at all.
I have this same problem too- Except for me in chapter red flags it showed 75% affinity with Cain when i chose the red dress and in characters screen it decreases to 63/68
The percentage seems to mess up when i replay any episode in between that wasn't the last one i played
