Flowey's circle bullets attack disappear right before Susie uses Rude Buster. The reason for this? In Deltarune, grazing TP causes attacks to end sooner, meaning it's impossible for Flowey to hit Kris with this attack.
Why is Papyrus still setting up traps when Kris and Susie arrive in Snowdin? Because the time they were transported to Undertale is over two hours earlier than when Frisk fell down into the Underground.
In the Obliteration Route in Chapter 2, despite having enough LOVE to make enemies bleed, the Blue Cultist doesn't go down in one turn. That's because he's human, and humans aren't vulnerable to Killing Intent.
The only minor exception to this rule is when you're fighting Ness and Paula and attack them when you make them reluctant to fight you. But when you think about it, aren't you more likely to get killed by the enemy when you don't think they'll attack you?
Similarly to how Flowey will give the player hints on unlocking the True Pacifict ending if spared in a Neutral run of Undertale, he appears at the end of Deltatraveler's Chapter 2 update to tell the player about Hard Mode. And much like how the former was part of a ploy to gather all the monsters in one place so he could steal their souls, he has an ulterior motive here, too; by seperating Kris from both the player and Susie, he can overpower and capture them.
Speaking of Hard Mode, why can't Frisk use Red Buster with Susie when they also have a Determination SOUL to shine their power onto Susie? Well, Frisk and Susie barely know eachother. By the time you fight King in Deltarune Chapter 1, Kris and Susie have become friends and have weathered a whole adventure together. Frisk and Susie only just got out of the Ruins. Not to mention that Frisk likely wouldn't know how to shine their SOUL's power onto Susie given that they only use their power themselves in order to SAVE.
The cyan (light blue) SOUL mode makes you slow, but you can parry attacks if you press [Z] at the right moment. Sans uses cyan colored attacks (which do not hurt the player SOUL if the SOUL stays still) and he can dodge your attacks in Undertale's Genocide Route. Also, the cyan SOUL had the virtue of patience. See the connection?
How are Kris and their friends able to take on Porky's mech, which is an endgame boss in the original game, while under-leveled? Well, Ness and his friends don't know how to ACT, so they wouldn't know that exposing the eye would lower the boss' defense. The Delta Warriors on the other hand...
During the fight against Ness and Paula, once Paula Turns Red because of you killing Ness, a version of Megalovania starts playing. It's more of a Genius Bonus than anything, but Megalovania originated in an EarthBound hack made byToby Fox himself. Bonus points for the fact that the hack's version of Megalovania is often nicknamed "Dr. Andonuts' Rage", since Paula is utterly furious since reviving Ness with a horn of life failed and now Paula wants you dead.
Speaking of the boss against Ness and Paula, it's rather fitting that the theme of the first phase is called "Roadblock." After all, didn't you know Onett is famous for its roadblock happy police?
Fridge Horror
In Chapter 2's Obliteration Route, you end up killing Ness and Paula.note Not Kris, since there's the possibility for Ness and Paula to be spared by having Kris tell Ness how they feels that their life is outside of their control. No, it's you who killed Ness and Paula, since you're the one controlling Kris, and Kris knows this. This means that you made Giygas unstoppable since Giygas's only weakness is Paula's prayer. Not to mention that, in EarthBound proper, you have to pray alongside the entire EarthBound world in order for the prayer to work. You Bastard!
And, it may be possible to do that to Link and the Mario Bros in their chapters preventing them from saving their worlds.
