I thought I'd explain the recent chapter, if you haven't done it, read chapter 17 because it does the reveal for character M.

If you noticed, a mystery character talks to you at the end of almost every chapter, in the beginning they told you about the rules for the game, in recent chapter they started talking about how you're cheating their game. That was character M.

And in chapter 17, character M was revealed to be Bruce, and I feel like I should give an explanation on how he's been so aware of Y/n's situation, and why he's able to know.

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Your first hint that Bruce was character M was in Chapter 11. After he rescues you, he calls you by your full name, and reveals he knows your situation, BUT if you re-read the chapter, you'll see that he never explains how he knows.

When Alfred suggests why your 'current' memories were gone, Bruce goes into detail on the possibility that you made a deal with someone that caused you to lose your 'current' memories and gain you past ones. 

At the end of that chapter character M goes on a rant about how if wasn't their fault that you made the deal, it was your fault.

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Your second hint was the beginning of chapter 12, he tells Dick that you were born here, and that it meant you died in your last realm. 

How does he have so much information on reincarnation?

In chapter 12, Bruce is busy trying to defeat the Music Meister, so if you look at the end of the chapter, you'll notice that character M is not present. Chapter 13 does the same thing.

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In chapter 14 it's revealed that Bruce is working on something that will wipe Y/n's memories, which is now called 'The Cure'. At the end of that chapter, we learn that character M references themselves a M.

They go on a rant (Which was inspired by the narrator's speech in Stanly parable), this rant might of threw people off because the rant makes it seem like character M hates you. 

But I can assure you they don't.

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Why is Bruce so aware and obsessed? I should be clear this is not a fourth-wall break, I mean he's aware that you're not from his world.

Well, you have to remember the fact that you don't have your 'current' memories, so the answer to that question is related to your forgotten memories.

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I hope this wasn't too boring to read, and I hope this answers some questions and plot-holes.