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There are several costumes in chapter 5, varying just how naked Christine is in certain spots and changing one particular outfit later on. Chapter 1 is getting a new costume patched in in the next update.

The later chapters do refer to the earlier ones, such as Christine finding Jeanne's artbook.

As for Cassandra. Wow that sounds like a major spoiler!

No, as in, the answer is a major spoiler so I'm not going to answer it. You'll just have to play the game to find out.