He came from an AU called "X-Tale". His AU owner or creator JAKEInimation (Jael Peñaloza).
Cross has one white pupil and one red pupil, and a lightning bolt shaped "scar" under his right eye socket. His soul is half an upside-down white monster soul and half a red human soul.
He wears mostly black and white. His attire consists of a black jacket with a hood and several white attachments and white fluff on the hood, black shorts with white Xs on each leg and white shoes with Xs on them. He wears a gold locket and a black and white tattered scarf and fights with a giant knife.
Cross could take control of a victim by executing them temporarily to make the victim their puppet. Cross can also steal parts of universes with his blade. After getting his hands on X-tale Ghaster's creation 'Overwrite' He has the power to rewrite actions, characters, events or the whole cast if he wants to.
Cross and Chara stole half of Classic!Sans soul, which enables them to have Classic!Sans abilities. Cross has a Gaster blaster that has four eyes and red pupils that fires red laser beams insted of blue. He can use knife like weapons instead of bones. The knife ability most likely came from Chara and their determination.
Cross lived in a universe somewhat like Classic!Sans. He had friends and family. Chara destroyed his world and was somehow trapped within Cross's soul. His world had no resets or true resets so he stayed in that state for a long while. Ink has helped him control Chara, but Nightmare (Dreamtale) offered him somthing he couldnt resist. Getting his family, friends everything back. Since Ink told Cross a few days before he couldn't do this, Cross agreed to Nightmares plans. He left his AU and used Chara to 'hack' portals into other AUs where they would copy codes. However Cross soon became soft to the inhabitants of Underswap and without knowing, gave Chara all the room he needed to take full control of the body. The rest is still getting written in Underverse...