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Oh, it's not because of the game that I say that, but it's like this: First impressions are everything and introductions are everything. Thus, how one sees a series can largely be based on how they started it, especially if it left an impression of them.
Therefore, one who starts with days may have it firmly planted in their mind that Roxas is the centerpiece of the story, and that Riku is...well...evil and a complete d!ck. While one who plays Kingdom Hearts first will see Sora as the centerpiece and will think of Riku differently. It doesn't really matter what they know, it's because of how they are introduced that they will see things differently.