hello! its the magnificent sans here with my brother! papyrus. If you don't know what underswap is,its baseiclly the origanils just the personality swamped  ... example:
sans and papyrus
 
sans is more of a slackere and joketer

 papryus is more of a hard worker,and has a thing for becoming the head of the roal gaurd.

now for the swaps

papyrus is the slacker and the joke ster.

while sans is the hard working and becoming the head of the roal gaurd.
so I hope that helped a little bit if not I'll probly make a book or something ok ...bye!