You can learn a lot about a person from seeing how he lives, for example, if a man's house looks like a crack den, chances are he is a crackhead. What insights could I glean about Deltarune characters from how they arrange their personal living space? Cue the armchair psychologising!
Kris shares his sleeping quarters with Asriel, his room is furnished symmetrically, one side is Asriel's, and the other is his. Asriel's side of the room is richly decorated, his shelves are full of trophies and awards, starkly contrasting Kris' complete lack of any such things. Psychologically, I believe this hints at an inferiority complex towards Asriel. Every time Kris enters this room, he would see Asriel's trophy shelf in comparison to his own, it would serve as a daily reminder of his own shortcomings. This probably means that Kris is a person who is very hard on himself and dwells on his flaws. The rest of the details reinforce this: Asriel's wall is decorated with stars, referencing how Kris sees Asriel as "shining brightly like a star" while his is bare. Asriel has a clock on his drawer, clocks are objects which are used to keep time, symbolizing his willingness to rise early and be places on time, while Kris has no clock, he ignores the passage of time and doesn't care about being punctual. Asriel's lamp is on, while Kris' is off.
Asriel has a desktop computer, a piece of information technology representing how he's in touch with the bleeding edge of recent developments. In its place on his side of the room, Kris has a bird cage in a wagon. A bird trapped in this cage would be unable to fly away, it would have no choice but to go wherever the wagon takes it, this represents Kris' feelings of being trapped and having little agency over where life takes him. It's also worth noting that Asriel's side is slightly bigger than Kris'. This is representative of Kris relenting space to Asriel, meaning he thinks Asriel is more deserving than himself. The colors themselves are more saturated on Asriel's side, which makes Kris' side look dull in comparison.
It's also no coincidence that Asriel's side of the room is the left one. Because most people are right handed, they prefer sleeing on their left side so their right arm can stay free. Asriel would be able to sleep on his side and maintain awareness of the whole room, representing that he's aware of his place in life and the world, and that he's always dilligent and in control. Kris, meanwhile, would see only the wall, even if he turned around, his dominant arm would be trapped, tying back to his own lack of control.
There's also an element of love and reverence for Asriel, as Kris has made no attempt to interlope on Asriel's side of the room, despite his extended absence. He's left his belongings untouched, almost like a shrine in Asriel's honor.
Ralsei's designated bedroom in his own castle is almost completely base, except for the unlockable plushie. He has put a double set of curtains on the window. Curtains like this are an embellishment mainly for the sake of style. That he would put up fancy curtains like this before any other fittings hints that Ralsei might be someone who doesn't have his priorities in order, that he's prone to putting the carriage before the horse, so to speak. The lack of furniture itself could be for two reasons: It could be that he doesn't care about furnishing his room, or that he hasn't figured out how he should furnish it.
He cared enough to put up curtains, stylish curtains that do more than serve the function of blocking the window, so I believe it is because he doesn't know how, he wants to put something there, but doesn't know what he wants his room to look like. This clearly shows that Ralsei has an identity crisis, the lack of any kind of furniture in his room represents the lack of identity and belonging in his mind. If Ralsei slept in this room, he would sleep on the floor, which is uncomfortable, representing how he doesn't feel comfortable with himself.
His choice to prioritize curtains as the sole feature of his room represents his need for privacy and secrecy, and reveals that he's probably hiding many things, windows literally give insight into people's homes and lives, his desperation to block that insight shows that he doesn't want to reveal too much about himself to others. Or maybe it's the other way around, and the curtains are mainly to block himself from looking out, representing his unwillingness to see the outside and his desire to lock himself in his own world?
Unlike Ralsei, the barrenness of Susie's room more likely stems from her lacking the means to decorate it, rather than an unwillingness to. The room itself looks squalid, the wallpaper is tearing, and Susie appears not to have made any attempt to fix it. This clearly represents Susie's neglect of herself and her life. The decay of her room is a factor outside her control, the lack of any effort to remedy it shows that she doesn't take initiative, and would rather stay detached from the encroaching rot, also figuratively, it represents how she doesn't care what happens to her, she doesn't plan far into the future.
The poster on the wall is peeling off, it's something Susie could fix easily, but doesn't, and it reinforces the previous point about self-neglect. I speculate the poster might be covering up a bossmanjack style gamba sesh punch hole. The fact Susie made the effort to at least cover this up subconsciously means she wants to hide her aggression, even if she fails at it. The poster itself depicts a scene from a kaiju film. I'm not an expert at kaiju films, but if someone puts up a poster from a movie, it generally means they relate strongly to that movie. What does Susie see in this movie that she relates so strongly to? Could it be that she relates to the monster in the movie, the destructive force of nature that brings terror and destruction wherever it goes, symbolical of Susie's feelings of being a (literal)monster, destroying everything she gets near including her personal relationships, willing or unwilling? Or does she relate more to the people in the kaiju film, being at the mercy of a giant monster that could destroy them all with ease, representative of Susie feeling like she's at the mercy of her life circumstances? I think it's actually a little of both.
The shelf above the bed is the exact same sprite as the one in Kris' room, but there's nothing to contrast with here, it's just an empty shelf. The emptiness of the shelf just makes the room look even more empty than it would if there was no shelf, it emphasizes her feeling of general emptiness and lack of meaning.
The most interesting thing in the room is the chair in the middle of the room. The chair is turned to face Susie's bed. If someone were to sit in this chair while she slept, they would be watching her sleep. Most people would avoid putting a chair in this position for that reason, it's a "cuck chair". Susie, however, has chosen to install the chair in this way. It reveals Susie's subconscious desire to be seen. Since Susie holds her axe in her left hand, it probably means she's left handed, which means she would prefer to sleep on her right side. While laying in bed, Susie would be able to see the chair, revealing a willingness not just to be seen, but also to see and interact with others, all this probably means she feels very lonely. The chair's position in the middle of the room means that a person sitting in it would be in the center of the room and at the center of her attention, it hints at a gentler and more caring side, that she would be willing to put someone else in the center under the right conditions.
