In an official sense, there are two ways to see your passive run character. Either, scrappy and determined, taking hits and dying, restarting, etc. -OR- Alternatively, Frisk is just a really, really agile human.
Like, on a good enough pacifist run, Frisk would be effectively walking through the underground, and monsters would show up to attack and proceed to have their lives quickly and efficiently made better by this human who, despite their best attacks, is just completely untouchable. Like, when you have to clean Royal Guard 02’s armor, the screen for that is literally you dodging their attacks to wipe dirt off, as in, Frisk jumping and weaving around with a rag, polishing dirt off of this guy’s armor.
Or, playing with the greater dog while simultaneously dodging its attacks. You are playing with a dog who would literally kill you if you let it, which implies a certain level of multitasking.
Point is, extremely agile PC sounds like a lot of fun.