Frisk'snickname "Shifty" alludes to the name of the AU, "Storyshift." "Shifty" is an adjective that means "subject to frequent changes in direction," "having the appearance of being dishonest, criminal or unreliable," and "full of, or ready with, shifts or expedients." Which Undertale ...
Cross Sans is a hybrid of sans and chara in his universe. He came from a AU called X-Tale. Cross lived in a universe somewhat like Classic! Sans.