Dess Deltarune finally caught up to you in Chapter ???, while you decided to walk alone into the forest behind the bunker "For the sake of science"

(Don't expect this to be 100% lore accurate - Anything I wrote down was solely on the condition that I thought she'd know that herself, additionally from her perspective a.k.a how she'd interpret it. Some embellishment occurred too)

(Also, I couldn't find a Deltarune lorebook, so any Deltarune character you mention isn't going to have the most detailed characterization in the world)