Underfell is an Alternate Universe of the main timeline, changing every character's personalities and fighting style.
- Blackjack is submissive, wears a collar and is TinySatan's pet.
- TinySatan is much more dominant over her husband
- EC is much more cruel and lustful, she carries a knife at all times and is almost always aggravated
- GEC is very weak and paranoid, unable to fend for herself, evident by the many scars adorning her body
- Jasan is much more evil and aggressive towards others
- Skull is much more of an asshole toward his family, save for his wife
- Erica is an asshole and a bitch, especially toward her son and friends
- Zach takes after his father, being much more easily aggravated, especially toward his mother
- Ace and Georgia are actually much nicer towards each other and others, one of the few in Underfell
- Grace is much more easily aggravated and mean towards her siblings
- Future Jay is very aggressive and evil against others but is much more mild-mannered around family
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