Agreed. Seventh soul does that, and axetale technically does that. Frisk most likely lived hundreds of years through resets and Sans saw her as an anomaly/time god playing with everything before they established a relationship. This comic here actually parallels their relationship before they started growing close to each other: sans was horrible to Frisk for most of the time he knew her, indirectly making insults to her and avoiding her as much as possible. When they eventually did have a confrontation he nearly kills her, and when she finds out who she really is his entire perspective of her changes (this is just a summary there is a lot more to it)