Used to do these for a different fandom on my since-deleted sideblog, and since my brain has decided to pull me back into this one by the hair, allow me to present:
!!Rapid-Fire c!Karlnapity Headcanons!!
First up, species! c!Sapnap is a crossroads demon, c!Quackity is a classic bird-person style siren, and I see c!Karl as being a poppet of some sort. Partially because I wanted an excuse to give him some witchy motifs because I think he's neat, but primarily because it fits with the whole "plaything for a higher being, used to further the goals of something he cannot fully comprehend" vibe that he's got going on in the lore.
Re: Crossroads demon c!Sapnap: He can't use any of his magic for himself - hence why he doesn't just solve all of his problems that way. In order to use his magic, it has to be for someone else, and it has to cost them something of equal or greater value. I also 100% believe that c!Dream would've tried to sell him his soul for power on multiple occasions and gotten told no every time.
c!Sapnap did ballet as a kid, and while he enjoyed the "doing ballet" part, he did NOT enjoy the "interacting with ballet teachers" part. So he quit attending classes and is thus mostly self-taught.
I will forever be a "c!Bad is c!Sapnap's adoptive father" believer, and you can pry the idea of c!DemonDuo as a family from my cold dead hands.
Related to the above: I have no firm stance on who c!Quackity's parent[s] would be, but whoever raised that man did not hug him enough as a child and it fucking shows. I don’t know if that's even a headcanon or if it's just character analysis but either way I stand by it.
c!Karl does graffiti. Constantly. There is not a single flat surface in Kinoko Kingdom that is free of his art. And he's a founding member, so it's actually legal for him to do here - not that something as minor as "the law" ever stopped him, but it's still nice.
c!Quackity is the only person in this polycule who does not know how to sew at all, and that can be quite inconvenient for him at times. He does, however, have the option to use his inability to sew as an excuse to get right up in his fiancés' space whenever he "accidentally" tears one of his sleeves. Pro's and con's.
c!Quackity is the shortest, though not by very much; and thanks to his beanie, you can't really tell. c!Karl is shaped like a goddamn street lamp. His fiancés are at eye-level with his pecs, and are taking advantage of that 24/7. Free pillow babey let's go.
c!Karl colour-codes basically all of his belongings and the places that he stores them in the vain hope of not forgetting where he put his keys. He still forgets a solid 85% of the time, but it's progress.
All three of these men act like cats in their own way. c!Sapnap does that thing where if other cats pick them up by the scruff of the neck they just kinda,,,, bluescreen. c!Quackity likes to sit on or in literally anything that fits him. c!Karl will cause [very unserious] problems on purpose if he wants attention and isn't getting it.
These men do not have a concept of "mine" and "yours" anymore. Our house keys. Our potion stash. Our shirt. They would use each other's toothbrushes without even noticing if it weren't for the fact that their teeth have different needs due to being different shapes. They just quite simply do not care anymore.
Every time that c!Karl remembers something funny that happened, he brings it up instantly - partially because he likes making his fiancés laugh, and partially because he's happy to have actually remembered a thing.
They all think that they're the sole holder of the braincell. They're actually just playing hot potato with it 24/7. None of them control when it is their turn.