I'm imagining some things that Kris!reader, Frisk!reader and Chara!reader would do while traveling with the Chains.
Kris!reader doesn't really talk much and not showing much expression, they only show it through action(not a lot either so Frisk!reader have to help the Chain understanding Kris!reader more).
Frisk!reader showing a bit of expression like a happy, sad, etc. before returning to '-_-' and the most normal one to talk to out of the three.
Chara!reader having much expression and action than the other two but not the best person to get some advice from and maybe mischievous(they can be nice sometimes), but Frisk!reader are already used to them and Kris!reader doesn't really care at all so the Chain will have to get used to it.
Fighting monsters(Not Undertale or Deltarune cause Pacifist):
Kris!reader can change between their Overworld and Dark World outfit, Overworld outfit when they are not in battle, Dark World outfit when in battle. And since they are not in their world, they have no choice but fight back.
I don't know if Frisk!reader wants to fight back since they are from Pacifist route. So it is up to you either they have no choice but to fight back, tried to befriend them or not fight at all.
Chara!reader will definitely fight back when they'll going to fight a monster that is not from their world(Pacifist route) since they have a weapon that is useful for battle even though it's just a knife. But if it's Chara!reader from Genoc!de, let's just say... they'll have a great time. ( =
Weirdest thing they've done:
Frisk!reader haven't done anything weird.
When Chara!reader can't sleep, they'll sit up and stare at the distance with a '= )' face. If you see them like that at night, they'll stare at you back in silence until you look away(That's not weird, that's scary-).
I actually like the way you've characterized them, especially in their facial expressions and actions.
However, for the sake of fleshing this au out, I want to say that Frisk! doesn't necessarily have to be a pacifist to be that run. They can be a jerk but still not fight or they can fight but keep it a neutral run.
That being said, I also want to point out that if we're keeping Frisk as the pacifist, that not only would they hate it when the Chain attack monsters, I think they're character would serve to really highlight just how messed up their childhoods have been and what would technically be a normal reaction to danger.
Frisk got to stay a kid despite it all (I'm sticking with this idea, I know that it's easily arguable) and because Frisk is so young, I think that it wouldn't only cause tension with the older ones but with the younger ones too.
He was younger and could handle it. Why is Frisk the same age (or not) but not able to handle anything? Is Frisk just a baby? Weak? Sensitive?
Wind is a teenage boy, I don't think he'll have the patience to be gentle in regards to a fight. He takes pride in fighting and saving the day. What's Frisk's deal?
Chara kinda has to stay a Genocide run since that's the only one where they more or less have their physical form. (Stolen from Frisk but I digress). So they're going to have the keep the sharp objects away from them for most of the trip anyway.