He came from an AU called "X-Tale".
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.
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. He has a plan to get his world back, to steal from other AUs.