He is the first person that Rosa and Jack meet, he's turning up to be really damn cool, he has aspects for what sans was (Only the metal prosthetic arm) but makes him a unique character. Hollow earth is small compared to our legends of it but it has two sides. The Floor, and the ceiling. Since gravity is odd down there you can flip yourself to either side. But both sides want to take the other for recourses only found on one. The reason our skeleton friend (honestly more skeleton inspired if anything in terms of design) is wanted is because he well, illegally moves to each side constantly. But if there's anything the inhabitants hate is humans. They despise them for the pain they cause them. Anyone who sympathizes with Humans will be executed by both sides. Only a small group of these beings resist this law. So when Jack finds this odd world in search of refuge from the War that had just started he kills everything in his path due to Peismas influence during the LOOONNNGGG fall.

Only the resistance of the hate against humans survive. They are needed for the finale.

Side note any trace of Chara and Frisk have been removed there were hundreds of humans before and all were killed.