Frisk swings their legs, knocking their heels together. They don't know...if they care as much about Dr. Gaster's reaction. Which is probably mean of them. They're supposed to be nice. But they love S-4 and Sans-Serif a lot, and don't want them to have to deal with bad, dangerous stuff thanks to Gaster and other scientists.

"...yeah, but...that job was to fight me, wasn't it?" Or so Frisk assumes. They don't know how many SOULs the underground had at that point--not that it matters. They wouldn't want either of them to have to fight and kill a human. "The me in your timeline. And you...won't need to, if you're here."