The main antagonist in Chapter 1 will be Flowey. Here, unlike his version from the undertale, he is a more positive character. You can also see new trees from the locations in the background.

Fire...

Flowey in battle

GIGA Sans

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Papyrus and his best friend FLOWERY

In all chapters (maybe not) secret boss fights will be done as in the Undertale.

Chapter 1: Dedicated to nature

Chapter 2: Devoted to technology

Chapter 3: Dedicated to Slot machines

Chapter 4: Dedicated to heaven

Chapter 5: Dedicated to caves

Chapter 6: ?

Chapter 7: Dedicated to KINDNESS

Some of you might remember this from years back.

Secret boss in chapter 1