My Deltarune headcanons:

Susie is homeless
Kris is mute because of an accident when they were younger
Kris is also claustrophobic
Ralsei is very depressed without Kris and Susie and even more so after they leave
Ralsei becomes a fricken flower when he's DOWN or sleeping or way too stressed or any worse-than-usual negative feeling
Lancer hides all of his many MANY worries about being the heir to the throne and all his duties behind a facade of child adorableness and joy
Roulxs Kaard is more of a dad to Lancer than Spade King ever has been or ever will be and talks with the poor child about Lancer's royal worries
Spade King was a total happy softie before his Queen disappeared
Jevil constantly thinks about Seam, wondering when his friend will come to be 'free' with him
Seam is slowly getting corrupted by whatever corrupted Jevil