Headcannons for Undertale and it's AUs:
UnderTale & UnderFell Chara narrates. I think that's a thing, narrator! Chara, while in UnderSwap is Frisk narrating things. I imagine Chara has a slight higher voice pitch than Frisk, making it a bit easier to tell that Chara's a female while Frisk has a monotonic pitch that makes it harder to tell what gender she is.
UnderSwap Chara invites everyone for tea and chocolate while UnderSwap Frisk asks for sweets and coffee.
UnderFell Chara talks in third person because she can. Frisk in the same universe flirts in puns.
UnderTale & UnderFell Chara is younger than their Frisks while UnderSwap Chara is older than her Frisk.
US Frisk can beat the crap outta you using nothing but her fists.
Although every Frisk has a monotone voice, US Frisk has the deepest one due to being "done with your shit".
UnderFell Frisk, believe it or not, curses a lot more than US Frisk.
Classic Frisk seems to talk in memes occasionally.
UnderFell Sans is a Tsundere while his brother is a kami-dere(treat me like a god, human!)
US Sans is more mature and knows what "bedroom related activities" mean. (I'm not stupid, brother. I know what monster heat is and I'm quite sure you seem to get that when you're around Cha-)
US Papyrus is a horrible cook while US Sans is a decent one.
Chara bakes cupcakes while Frisk bakes cakes. Layered. With frosting.
Every Frisk has a sweet tooth.
US Sans hugs US Frisk in the back so he won't hit her breast. US Papyrus likes to walk behind Chara for reasons.
Classic Papyrus carries Chara in a piggy back ride. Since US Frisk is taller than US Sans, he carries her bridal style. UF Papyrus carries Chara over his shoulder.
Did I mention there's a Swapfell version? Eh, nvm. It'll take too long