ohhh i love this! what a fun question to think about!
okay so… that wording is really fun too, because it means everything that happens beforehand also stays exactly the same until the moment they ‘fall’. and we’ll say for the sake of having each world function that they still have a SOUL, but it’s fueled by their own will rather than the will of the player.
so, for kris:
they just entered the closet with susie. they were hesitant to go, and then when they fall… if we’re not in control, than whatever ‘force’ was controlling them is absent. because frisk is alone at the start of UT, lets say kris also begins by themself (susie doesn’t come along with them).
so they woke up being controlled by something, unable to manipulate their own body for the whole morning, then they get bullied and cant seem to defend themself, then they enter a strange closet that turns into a pit, and wind up in some dark cavernous place all alone. i’d say they probably feel pretty freaked out!
then we consider the fact that, if everything leading up to that point is exactly the same, kris will end up meeting characters who look exactly like their family, and many of the monsters will resemble people they know from the neighborhood but just slightly… off. same names and everything.
i feel like kris wouldn’t trust flowey. they’d be at first comforted, then freaked out by meeting a toriel who doesn’t recognize them, but trust her ultimately. they’d also want to leave, and would be reluctant to fight her. i don’t think kris would kill any of the monsters, especially the very small skiddish ones they encounter at the start. i think when toriel tells them to be careful and to be good they would take it to heart, and by then kris would understand that they don’t seem to HAVE to hurt anyone to get by.
im gonna try and generalize the rest from here so it’s not a total step-by-step summary of how i think things would go, hah, but i think in essence… kris COULD manage a true pacifist ending. i think they’d be very wary of everything/one, but ultimately they would enjoy exploring this weird world full of very friendly creatures who seem comparatively less cynical to some of their counterparts in deltarune. they’d have very mixed feelings about fighting asgore, but some things in DR hint that their feelings toward asgore aren’t wholly positive, and i think kris would be willing to FIGHT (and then SPARE) asgore as required.
i think the biggest issue might be whether or not kris shows flowey MERCY after that fight, but given that kris grew up with an alternate version of asriel, they may see enough of their brother in flowey (even without knowing they’re the same) to show MERCY to him.
and then the rest goes as it does. dunno if kris is as much of a shipper as frisk certainly is, but they’d be down for helping alphys and undyne. they’d be very freaked out by the dark lab i think, and everything leading to the end would be SUPER WILD for them.
honestly? going at undertale from this perspective, it could very well read as a sort of ‘coming of age/exploration of inner turmoil’ sort of story. this is something of a given, considering the way these two worlds are structured to mirror one another. but think about it: their parents are divorced, now literally separated by an entire world stretched between them. it has themes of power and choice, only this time kris actually has the ability to choose. for the first time (?) in their life, their actions hold real consequences - their choices matter! and if we’re to speculate that the player taking ‘control’ of kris is a frequent thing that happens, this may be one of the few times in their life they’ve really been able to explore themselves and an environment on their own terms, completely. no stuffy small town, familiar but unfamiliar faces, no one and nothing to fall back on… they really gotta navigate this place on their own, grow as a person. it even works with asriel being ‘absent’ throughout most of the game; that could read as their conflicted feelings about their older brother, the fact that he left for college, etc.
all in all, i’ll say we can stay optimistic; i have a feeling kris could achieve the true pacifist ending naturally, or if not, at least a neutral ending. if we wanna tie things up neatly and play by the same rules, they’d then exit the underground back in the supply closet with susie, who would have gone on her adventure with…
frisk:
this one would be a lot more simple, given the nature of the game/world (and the length of DR). frisk is exploring mt. ebott, falls down a hole, and wakes up looking different in some dark cavernous place all alone. they walk until they find susie, and HONESTLY it may be a toss-up whether or not susie sees frisk and thinks they’re kris. obviously frisk is much smaller, but frisk isn’t the only one who got a wardrobe change. susie might assume frisk’s younger appearance is part of this weird place and whatever changed their clothes and gave them weapons, etc.
for the sake of simplicity, let’s go with that route: susie assumes frisk is just kris, and they look different and act a little differently because of Weird Magic World Shenanigans. frisk would probably just go along with whatever susie said with little to no comment anyway, so it works. i’d say unless you wanna go hard with the ‘frisk is nonverbal’ headcanons, there wouldn’t be a way for susie to know and frisk just wouldn’t correct her. at most, they’d be surprised she keeps calling them the wrong name, but a name that is kinda almost similar…
anyway! from then on i think the game would be mostly the same. go along with the prophecy, try not to fight anyone, etc. Since DR’s world works more in a ‘your choices don’t matter’ context, frisk may naturally be at odds with some of what happens… there may be moments where they really want to make a choice, to change something, but end up feeling powerless, which is a feeling we don’t often experience in undertale (which is kind of the point).
however, we don’t know anything about frisk’s background, unlike kris (which i’ve talked about before). we can assume they’re a goofy, playful, friendly, kind, helpful, creative, empathetic, passionate kid, but we don’t know what their home situation was/is like, or how they’d respond to an environment where they can’t utilize their free will often. i suppose you could say, in a way, they end up in the sort of position kris starts in before they enter dark world, when we were controlling them: being pulled along a set path, unable to deviate or make meaningful choices.
sure, frisk gets to make some friends, they can still SPARE others and show MERCY, and the fact that they could FIGHT or try to do real harm to others isn’t relevant because we know frisk, as a character, wouldn’t do that (again, WE would do that, not frisk). But it doesn’t matter, even if they don’t utilize that choice it wouldn’t matter, and that’s the point of DR.
I feel like the end would be much sadder for frisk. they defeat the spade king, they say goodbye to ralsei, they do not understand susie’s strange reaction to seeing ralsei’s face even in the slightest. they go with their new friend back…
home?
and they are alone, all over again.