LogFAQs > #985481472

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I beat the final video game boss and got the shadow mantle. Where do I fight this secret boss, and did I miss anything cool by having NES Kris walk off screen? I didn't think to have him walk toward real Kris and sword strike him until he was already gone.

First question, it is basically impossible to miss, don't worry about it and keep playing
(specifically, you've done the missable bit already, you can't miss the rest of it)

For the second question, there is an alternate scene if you actually have the 8-bit Kris walk back UP, you actually have full control of them during the following dialogue with Suzie after it walks away. If you do it'll interrupt that cutscene and play different dialogue, but nothing crucial it's just a jokey easter egg. You can look it up if you want.