Mono is the Thin Man in Little Nightmares 2.
He becomes the Thin Man in Little Nightmares 2 thanks to Six’s betrayal, and this makes Mono’s transition to the dark side utterly heartbreaking.
It’s hinted at throughout the game’s duration that the two are connected thanks to their shared power over television signals.
This suggests that a time loop could be involved, and that the Thin Man is merely trying to save his original self from helping Six and thus eventually being betrayed by her.
It also raises questions about whether Little Nightmares 2 is a prequel rather than a sequel. This is because Six doesn’t wear her iconic yellow coat from the original game at the beginning of 2, as well as because Six doesn’t appear to have her powers earned at the end of the original game.
Although Little Nightmares 2 was said to be a sequel, there’s a secret ending which does lend credence to the theory it’s a prequel.
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