AU: Derek and Casey dated before their parents were even dating. When their parents announced their engagement, it came as a huge surprise to both Derek and Casey, but because Casey wanted her mother happy, she allowed her mom and George to get married—which had surprised Derek, because he wasn’t going to allow it. Casey convinced Derek that the two wouldn’t last anyway, so why not allow their parents to be happy? Derek on the other hand felt like it had all been decided for him and he was unhappy. After their parents got married, all Derek did was irritate and annoy Casey which was his way of venting his bottled feelings—and he enjoyed the reaction he’d elicit from his former ex, now step-sister.. This took Casey by surprise because the Derek she knew was not the Derek she knows now. In the end, it all comes down to one thing—they still have feelings for each other, but Casey would never cross the line, even if it means she could lose a valuable friendship.
"Derek, this needs to stop. The old videos need to go, you need to delete everything unless you want Mom and George to find out about our past. What’s wrong with you? You were different, so what happened?"
”—What happened was you and your decision. What happened was this marriage. You might have thought we wouldn’t have worked out, but you were wrong. You’re still wrong because I love you. We could have worked out but you chose for me and now we’re both miserable.” There was a short pause and through gritted teeth, Derek said what he thought he’d never say.
”I will never forgive you Casey.”
He watched as she turned around and walked away. He knew he had hurt her, but she had hurt him more. He knew she still loved him, but she decided their ending for them. Who was to say he would ever make it easy for her?