The Roaring Knight, often referred to simply as the Knight, is the main antagonist of Deltarune. It appears as a thin humanoid figure clad in pitch-black armor with a white outline. It wears a diamond-shaped helm with deer antlers and a slit on its face that can shift between a visor and a mouth, pointed shoulder pauldrons, white holes through its hands, thin waist with sharpened hips, and long legs ending in sharp points. It can change its shape at will, morphing into either a sphere or a bird-like form as a means of transport, or shifting its visor into a fanged maw and opening its chest to reveal the eye within the chest cavity.

Little is known of the Knight's personality or deeper motivations. What is known is that its mission is to initiate the Roaring, an apocalyptic event foretold by the Prophecy that, if not stopped, will result in the unleashing of gargantuan Titans that will destroy both the Light and Dark Worlds and the eventual deaths of their inhabitants. The Knight is ruthless in the pursuit of its goal, committing acts of extreme violence against anyone who would try to impede it, regardless of if they are enemy or servant. It often acts like a mindless, feral beast, yet it has also shown to be arrogant towards its enemies, even openly laughing at Susie when she claims that sealing the Third Sanctuary's Dark Fountain would be the end for the Knight and its plan. It is the creator of the majority of known Dark Fountains, and is thus responsible (whether directly or indirectly) for the motivations of King, Queen, Jevil, and Tenna.

The true identity of the Knight is as of yet unconfirmed. However, it is presumed to be a Lightner, as only Lightners have the capability to open Dark Fountains. There are multiple theories as to whom the Knight really is, though none of them truly matter until more concrete evidence is provided by future chapters.