In Deltarune, Asriel absorbed Dess' soul and became the Roaring Knight.
In Deltarune, soul absorption has not been mentioned yet. Maybe in this parallel universe, monsters can absorb each other's souls. I like both the "Dess is the Knight" and the "Asriel is the Knight" theories, so soul absorption would reconcile the evidence for both.
I think soul absorption works differently in Deltarune though. Fusing two monster souls wouldn't work under Undertale's rules, so the game wouldn't know what to do with Dess, which removes her doppelganger from the game.
The narrative matches up with how soul absorption is discovered in Undertale. Long ago, Asriel and another kid wanted to cross the barrier (Undertale) or the lake (Deltarune), so they made a plan to become the Knight. In the present day, the protagonist (Kris or Frisk) is faced with the same question: absorb Susie's soul, or try to figure out something else? (Weird Route? True Pacifist Route?)
In a different theory, Sans uses a machine to bring his punchcard from chapter 5 to the world of Undertale, which is stored in his basement. If Deltarune is the prequel, then something happens in chapters 6 or 7 to Susie in Deltarune that turns "Suzy" (her Undertale doppelganger) into a Goner. What happens to Susie? Kris absorbs her soul, making her disappear.
In the "Neutral Route," Kris absorbs Susie's soul. In the "Weird Route," Kris crosses the "barrier" (the lake). In the "Sword Route," Kris uses EXP instead of soul absorption in a vain attempt to bring Susie with them over to Undertale. Since all endings are theoretically possible, Undertale - the game - chooses to make Susie a Goner instead, and save itself the headache.
