Rude Buster is a spell that Susie knows from the moment we meet her in Chapter 1. It costs 50% TP, and, as the description tells us, it inflicts moderate "rude" element damage to one foe, and uses both Attack and Magic in its calculation. It considers defense in its calculation, and it scales better with Attack than it does Magic.

I want to compare this spell to another spell, Iceshock.

Iceshock inflicts "magical ICE damage" to one enemy, and costs 16% TP. It scales purely with the Magic stat, and is unaffected by her Attack stat. It also cuts through defense. On the surface, Iceshock is generally the better spell, it would seem. It might not deal as much damage at first, but it's significantly cheaper than Rude Buster, ignores defense, and scales like crazy when Noelle becomes Stronger.

While this is speculative, it almost comes off as if Iceshock is being more 'properly' cast. It's described as 'magical', it seemingly instantly surrounds an enemy rather than needing to be aimed, it ignores defense entirely. But... I think there's a bit more to it than that. Noelle's magic is kind of distinct from Noelle herself in a way. There's some level of detachment. There's distance. This is (partially) why we're able to manipulate her so easily into getting stronger. It's easy to not think about it. They're just enemies. Etc. But Rude Buster? It's a direct extension of Susie herself. She might not be directly naturally talented with Magic, but hell if that'll stop her. She channels everything she has into her axe and sends it out as a bolt of rude energy.

My point is this.

Iceshock deals perfect, magical damage to an enemy, piercing defense. It's better than a Susie crit at first, and it scales drastically. It's simple, and it's cost efficient.

Rude Buster is a direct extension of Susie herself. It's her raw emotion channeled into a single attack. It's her willpower, her resolve, her hope, all imbued into one little spell. Rude Buster as a spell is, either symbolically or literally, Susie's resolve.

This is why it is a "Rude" buster. What does it mean to be Rude? To be impolite. To not follow the rules, the expectations. And, if there's one thing Susie excels at, it's breaking every single expectation anyone might have for her. Is it truly by chance that, out of everyone in the party, it is Susie who talks back to the Roaring Knight? Is it truly by chance that the only party member who can actually do anything of substance against The Knight is Susie, with Rude Buster? Kris is (in most circumstances) holding back. Susie and Ralsei are able to deal chip damage. But Rude Buster, through sheer force alone, overcomes the Knight's defense, not by being magic or anything like that, but simply because it's that good.

Consider also The Titan. Everything seems bleak, the Titan can regenerate, and there's nothing we can do. So what do we do? We call upon Rude Buster. Technically it's "Dual Buster", but...

Susie and Gerson are both clearly casting variations of Rude Buster here.

And it works. And, lets think on that for a moment. Gerson also knows a version of Rude Buster. ...Why?

It's not like Gerson doesn't have his own magical attacks he could have used here, right? They could've easily done something else for this. But... No. Gerson casts his own Rude Buster. Why? Well, think about what Gerson stands for. He believes, in the same way Susie does, that the Prophecy isn't all it's cracked up to be. He believes that it can blind you, that it's better to read between the lines. As a Secret Boss, his philosophy is "I don't care".

So, to me, at least, it makes perfect sense that he would also know Rude Buster. Because, just like Susie, when confronted with the fate of the universe in bold text, he simply laughs it off.

This is also, I believe, why Gerson is the only character who can outright reflect Rude Buster.

Because while Gerson might not have the same resolve to change fate as Susie does, he is driven in a similar way.

The Devilsknife reduces Rude Buster's TP Cost. Why? Is it just because logically a jevil-turned-scythe would be good at channeling Magic? No. Think about what Jevil stands for. He believes that because his choices do not matter, he is free of consequence. He can "do anything", because his choices are irrelevant. If he could somehow be punished, then, well, his choices would've mattered, then, wouldn't they? And he knows that's not true. So he does whatever he damn well pleases. Obviously, Jevil and Susie are not really comparable- Jevil fully accepts that Fate is unbreakable, and Susie very much seems to think The Prophecy is bogus by the end of Chapter 4.

But, I think the throughline is there. Devilsknife makes Rude Buster easier to cast, because by nature, Jevil is already used to doing whatever he wants and ignoring the 'rules'. I hope I've made my case clear. But there's even more.

This may well be where I lose you, to be clear, so I hope you take everything I've said about Rude Buster as its own thing, and consider the rest of what I have to say as an extension of that. If you don't believe what I'm about to say, that's totally fine. Without further ado... Let us consider... The Prophecy.

The Second Hero of the Prophecy is "The Girl, with Hope crossed on her heart." As many before me have pointed out, this depiction... does not quite look like Susie. The weapon is wrong. This is a much longer discussion and I don't think I can quite fit it into this post, but, in essence, I believe that this was supposed to be Noelle Holiday. Noelle actually can equip a few swords as of Chapters 3-4, surprisingly. She can equip the Jingleblade and the Blackshard. However, I don't believe that Susie is "not" the second hero. I believe that The Prophecy has been tampered with. Or, at least, reinterpretted. Think back to what Gerson said. Stories can be changed. They can be retold.

Cat Petterz the RPG is a ripoff of Dragon Blazers, which is a retelling of Lord of the Hammer, which is a retelling of The Prophecy, which is a retelling of DELTARUNE.

I believe that this sort of thing is happening to the prophecy itself. I believe that Gaster, for one reason or another, changed the prophecy, replacing "The Second Hero, The Girl" with Susie. This sort of rules lawyering is possible because Susie is also referred to later in the prophecy:

We know that this is Susie because Rude Buster is being used to identify her. This image is even called "Rude Buster" internally. I believe the original prophecy was introducing Susie as a different "The Girl". However, because it technically uses the same term to refer both to Noelle and Susie, their roles can be altered. And so, Susie, through Gaster's intervention, became the Second Hero.

But why? Why was it important to make Susie involved? I believe it's simply because of who Susie is. When confronted with fate, Susie laughs it off. She won't let it happen. Wheras someone like Noelle would try and accept it for what it is, most likely, Susie outright refuses to play by the rules. And this gets us back into Rude Buster. Rude Buster is important. It's important enough to be the name of the battle theme, it's important enough to be what symbolizes Susie most directly in the prophecy. I believe that Rude Buster, and what it represents, is why Gaster chose Susie. Noelle might, in some sense, have "hope crossed on her heart". But it'd be passive hope. Wistful hope. Susie has active hope. With every fiber of her being, she has that hope. She inspires that hope in others. It is, as Ralsei puts it, infectious. She infects herself with hope, and grows it.

She infects Ralsei with hope.

She gives Tenna hope.

And, though this is more of a stretch, she even, indirectly, infects Seam with hope.

Remember that the only reason we could even potentially defeat The Knight is through Susie's perseverance, and Susie has to defeat Gerson, (who Seam is talking about here) on her own. Remember what Gerson told us.

Susie has the White Pen, that can draw over the dark pages of fate, known as Hope.

I believe that Gaster picked up his own pen, to transform Deltarune into his deltarune, one where Susie steps up to bat as the second hero of prophecy...