The Roaring Knight will not be the primary antagonist of Deltarune, but instead someone else. In Deltarune's parallel story, Undertale, Asgore is set up to be the primary antagonist and is foreshadowed throughout the game, but we never meet him. When traversing through New Home, we once again speak to Flowey, who we met during the intro, and it's revealed that he's the true villain of the story, not Asgore.
I propose something similar is happening in Deltarune.
The Roaring Knight is set up to be the main antagonist throughout the game: they are spoken about by multiple characters, and is typically seen as evil, exactly like what happened with Asgore. As we all know, though, Flowey's the actual villain. Who we meet at the beginning. Who else do we meet at the beginning? Gaster.
In entry 17, he speaks of an invention which is "dark, darker, yet darker" and "photon readings... negative," presumably the invention Gaster fell into that caused him to scatter across time and space Gaster follower #2 speaks about.
In Deltarune chapter 3, Ralsei explains how the dark world works, saying: "What if it became even darker? Darker than dark. What if we could take away the light that wasn't there?" Directly paralleling entry 17, which means the invention Gaster fell into was a dark world.
From this dialogue, we can gather that pieces of Gaster are scattered throughout dark worlds, which is why he wants to bring about the roaring. If everything is a dark world, all of his pieces will be in once place. How would he put himself back together, though?
Based on Gaster's chapter 4 dialogue: "How much longer now...? My Delta Rune?" he also wants to get his hands on that. And why would he want that? To reform himself.