The face said "I will fight you, over and over again, as long as it takes, until I can kill you." The hands however, begged and pleaded for help, guilty and shaking.
Sans was used to reading faces. Most skeletons didn't have far too much by way of facial expressions. Though there were a few like his brother, gifted with the ability to express themselves with ease, most skeletons had a difficult time doing so, and had to learn to read every subtlety in a face. Coupled with a human's vast range of facial features, for a smart skeleton like Sans, who had encountered humans more than once before, reading humans through their faces was child's play.
Hands? Not so much. And yet, Sans couldn't help but find something familiar about the way the kid signed.
This language of motions and gestures was not too uncommon in the Underground, as some monsters were born without mouths or ears, but this... this was different.
Most monsters who did have two hands, each with five fingers, did not tend to use it. Monsters with hands tended to have ears and mouths as well, and generally lived with other monsters that did, designing infrastructure that took advantage of their dexterity and opposable thumbs.
The language itself changed a little, depending on how many and what sort of limbs and appendages a monster had, much akin to dialects, but a basic knowledge of the language in general rendered even the strangest versions of it fairly comprehensible, give or take.
And yet, Sans understood each motion perfectly. As if he'd studied this particular dialect, even used it himself, perhaps, his whole life. So as he said "kids like you... Should be burning in hell," he asked the child how he could help, his fingers naturally falling into place, even as he used his magic to attack.
The kid grabbed their knife and lunged at Sans, but the free hand had motioned to where the knife was headed. He didn't need the warning to dodge the blow, but the act suggested that something in the child was fighting their own actions.
Sans took his turn, and while the child dodged expertly around the bones and blasters, the human told him to keep watch, not let his guard down, and watch carefully. All things he already intended to do. But what he didn't expect? That they would expect him to trust them. And yet that is what the kid told him to do. Trust.
Another motion to where the knife was aiming. Another missed knife attack.
Sans told the kid about the timelines while he told them that trust would be difficult.
The child continued to dodge in silence, while emphatically demanding that Sans wear down their determination. They spoke in third person.
Sans watched their face carefully. Still murderous, psychopathic,. Determined. And yet...
"I know you're in there, buddy. c'mere."
Their face scowled, and there was barely enough time for a warning motion before the knife sliced air again.
"you look pissed. did i getcha?"
And so the fight continued. Turns passed. They continued to talk without speaking, while Sans uttered taunts, trying to get them to give up. Sans expressed fearing that he was becoming tired, that he wouldn't be able to make it. The child gave him encouragement, telling him to stay determined, even as they ate a piece of snowman to heal their wounds.
The child told them that their name was Frisk. That once, their determination gave them the power to befriend everyone. To save the world.
It wasn't until too late Frisk realized the determination wasn't really their own. But now, it was beginning to wear thin enough to let them have just enough control to speak like this.
And they hoped Sans would eradicate it.
Sans, despite his wariness and doubts, believed the kid. He wasn't too surprised. Enough determination could manipulate time itself, but so much determination was unlikely to be natural.
Eventually, Sans reached his limit, and he was forced to use his special attack or collapse where he stood.
And despite his will, he couldn't stay awake forever. Slowly, he dozed off. Until they attacked again. "Did you" another warning "really think" a second dodge, "that you could actually" a third warning, "hit me" a third dodge, "like that?"
If the kid was planning to betray him, that would have been the perfect time. Sans' grin widened almost imperceptibly and his eye flared brighter than it did before. This proof, the evidence that he could trust the child who spoke in hand... It filled him with determination.
"let's finish this, kid."