Hmm what do you mean about " non linear progression " does it is story can be changed based on decision :? With me it a " - 1 point " :v
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There's a main storyline and each girl that's important has her own storyline to follow-through mostly independently from each other, so in theory you can focus on the girls you like first. There only one major decision so far in the game with Ulst, but honestly not really that nerve wrecking. There other that are minor ones like how you gather information for a quest.