Y/n is from krypton like Clark, y/n and Clark were best friends and his parents jor-el and sara and y/n's parents nor-el and mary were close friends as well which is also why y/n and clark hung out so much together back on krypton before everything went wrong. Y/n and Clark pods were sent down to earth at the same time but y/n's got knocked lose and was sent to a different part of earth separating her and clark.