(There is no particular order or rules for those, they're just random headcannons that I thought about.)

Error:

~can make random computer noises and made windows start up sound at least once.

~knows many languages, binary code included

~terrifyingly good at spider crawling (especially scary when he does it on the ceiling)

~likes to sit on the ceiling, how he stayes attached to it is a mystery

~sometimes uses his strings to move around a la spiderman

~he's secretly a cat or a spider, it's honestly a terrifying mix

~scared of everything and anything

~once during interrogation:

Ink: You won't make me say anything!

Nightmare: Oh really? Error could you do the thing?

Error: *makes windows start up sound VERY loudly*

Nightmare: He can do this of hours so you better start talking.
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Nightmare:

~he's really tool, like at least 2 meters tool

~knows at least 2 other languages, sometimes he says something random in different language and looks at the fear in peoples eyes as they think he just threaten them but in reality he said ''This is very good butter.'' or some shit like that

~hates water, it melts away his goop

~looks like he can kill you but is really an marshmallow, he's still gonna kill you if you piss him of tho

~he may or may not secretly be a cat

~loves cuddles, but don't tell anyone unless you want his tentacles in your chest

~has a soft spot for tiny and fluffy things, probably the reason why he likes Cross so much

~loves to read, especially romances and murder mysteries

~ ''How could you!'' -Nightmare after a character in a book did something stupid
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Killer:

~stressed, depressed and emotional mess

~likes to annoy Nightmare and test his limits

~he's both sadist and masochist

~likes chocolate only slightly less than Cross

~it's almost impossible to fluster this man, but when someone get him flustered his face looks like a tomato

~ ''I came here to annoy Nightmare and eat chocolate. And I'm all out of chocolate.'' -Killer
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Dust:

~likes to talk to the voices and his dead brother's ghost

~sometimes makes decisions by making the voices vote

~has lock picking at level 100 and is suspiciously good at walking through vents and finding hidden passages

~one second he's wandering about the meaning of life and next he's on castle's roof wandering if he can do triple backflip without hurting himself to much (surprisingly he managed to do it and only twisted his ankle)

~existential crisis 24/7

~ ''How many expired eggs do you think I'll be able to eat before throwing up?'' -top 5 quotes Dust said before disaster
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Horror:

~hates water just as much as Nightmare, it gets into the hole in his skull and that's a big no no

~if insults and curses were a language he'd be fluent

~always hungry, there was never time when Horror wasn't hungry

~he's tool, just slightly shorter than Nightmare

~he's actually a great cook but if you try some shit with him he'll poison it or make it the most disgusting thing you ever eaten

~likes to chew on other gang members limbs, they weren't found of it at first but you get used to it after a while

~any food is a good food, he'll eat anything, and I mean ANYTHING

~ ''Finally, some good fucking food.'' -Horror eating literal trash
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Lust:

~he's realllllllyy flexible, it's almost scary

~great singer, but he thinks he's just average

~dancing skills even Dancetale Sans is jealous about

~knows one foreign language ,but no one knows what this language is, he also knows a lot of random words from different languages and uses them sometimes to get everyone confused

~anxiety level: 243%

~he calls everybody darling or sweetheart, it's just his thing

~he's in Nightmare's gang, fight me

~ ''Listen darling you're my friend and I love you but you need to stop.'' -Lust after Killer was almost murder by pissed of Nightmare
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Cross:

~flufffffffyyyyyy

~totally a cat it's not even a secret, every one knows it

~knows Spanish and probably one other language

~he and Nightmare have a pack of cats hidden in the mansion, they usually keep them in the room they call 'Chamber of Purrs'

~he tries to stop gang from being stupid but has a hard time stopping himself from being stupid

~awkward gay vibes

~ ''So like... I like dudes... you like dudes... could we like... maybe... fuck!'' -Cross trying to ask Nightmare out, Nightmare took it as an invitation
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Geno:

~the mom friend

~he can curse people out in Russian, otherwise he doesn't know other languages

~always tired

~the guy who doesn't think he's gay, but everyone else think he's gay, and then he realises he actually is gay

~the only one who can stop the chaos in Nightmare's castle

~eather neutral or part of Nightmare's gang

~ ''Who hid the body in the basement. I'm not mad, just disappointed.'' -Geno
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Reaper:

~he's already tall but he always floats few centimetres above the ground

~uses his scythe as a cane when he's tired

~the only straight thing about him is the coffee he drinks

~doesn't speak any foreign languages but knows a bunch of random Latin words

~somehow has worse death glare than Geno

~Neutral or part of the Nightmare's gang

~ ''More espresso less depersso.'' -Reaper after 18th cup of straight black coffee
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Dream:

~But what if he was asexual? Well now he is.

~he knows few languages but they're all old and no one uses them anymore

~always acts happy but is actually really depressed

~tries to make everyone happy and forgets he deserves to be happy as well

~he's older than he looks

~the only reason why the Star Sanses still exist

~ ''Blue I know you love to ridicule Ink but now is not the time.'' -Dream during spy mission
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Blue:

~he may be a part of Star Sanses but he still is friends with all of Nightmare's gang and visits them frequently

~mainly a healer but not 'cus he can't fight, he just doesn't want to fight with his friends

~the only reason why he's still a Star Sans and puts up with Ink's crap is Dream

~has high fashion sense

~probably bisexual

~has a blackmail on everyone in the multivese, and he isn't afraid to use it

~A typical exchange between Ink and Blue:

Blue: Could you stop doing that?

Ink: But I'm not doing anything???

Blue: You're breathing.
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Ink:

~eather one of the gayest gays to ever gay or pansexual

~a bit bipolar

~knows French but only because someone told him it's a language of love and he thought it would give him great flirting opportunities, only later did he realise none of the people he knows know French

~can go from 'I'm having a great day.' to 'I'm going to commit homicide!' and back in 5 seconds

~the shortest Sans in the multivese, even Blue is taller

~IQ: -135 or less

~ ''I'm just doing what fortune cookie said. Who am I to stand in the way of fate?'' -Ink as he tries to light Star Sanses base on fire
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Fresh:

~he flipping screams gay

~furbies are his religion

~because he can't feel he sometimes wears clothing inappropriate to the weather, like tank top and shorts in the middle of winter or dressing up for Antarctica in August

~ 'I ride my heelies to escape my feelies' vibes

~wears few smaller hats under his normal hat, no one knows why

~ ''Hi, would you like to join my cult?'' -Fresh shoving furby in someone's face

(And to end it all a pic of my cat that nobody asked for.)

(Bye~)