Cross sans is from an au called x-tale. He lived in a universe which was a bit like classic sans. He had friends and family. Cross chara destroyed his world and was somehow trapped in cross' soul. His world had no resets or true resets and it stayed like that for a long time. Ink helped him control chara and... well... you can watch underverse to find the rest out yourself.

Cross can take control over a victim by executing the temporarily to make them his puppet. He can also steal parts of universes with his blade. After getting his hands on X-tale Gaster's creation, overwrite, cross can rewrite actions, characters, events or even whole cast if he wants to.  Cross and chara stole half of classic's soul which let's the duo have classic's abilities. Cross has a Gaster blaster that four red pupils that fires red laser beams. He can also use knives like weapons instead of bones.