underfell is basically edgy undertale, with nice characters being evil and evil characters being nice, so flowey guides you, chara appears after pacifist, goat mom tries to kill you so you cannot leave, sans is basically edgier, papyrus is evil af, undyne is a full on edge lord, alphys is a mad scientist, mettaton wants to be evil to please alphys but is insecure and too nice, napstablook is really rude, goat dad gave up on the underground, which caused anarchy, but still wants your soul, goatbro becomes edgy goatbro, chara wants to restart the timeline to befriend everyone, and etc.
