This probably sounds like the set up for every antagonistic couple in every teen movie you've ever seen, and on the surface, Derek and Casey are very much that couple.The 'we fight like an old married couple, look how our glares are filled with desire' couple.Prior to this relationship, he was rumored to be in a relationship with Ashley Leggat and Shadia Simmons.However, the rumor was not confirmed as he remained silent about it.Derek/Casey could be considered the fandom's OTP, as nearly all the fans like the pairing even when the network & creator's best efforts in the contrary.Canonically, they fight and prank each other a lot, however despite all of that - deep down they care for one another, which is one of the main reasons fans ship it so much.
Casey/Derek is a non-canon pairing of Casey Mc Donald and Derek Venturi from the television show, Life With Derek and the TV-made film (which is a sequel to the series), Vacation With Derek.It was especially true for the character Truman in Season 4 as well as some displeasure at Emily becoming Derek's girlfriend in Season 4. Sugah66 describes her feelings in a open letter to Daphne: "... (Ridiculous in the kind of way that seriously angers me - I mean, did you even watch the past four seasons? Seeing that much sexual tension between two step siblings is intriguing. It's on the Disney channel, which adds to the intriguing, dirty factor.It's fun to look for glances and too much touching, fun to read between the lines of their fighting.Because of this they often add in "significant" others for Casey and Derek each which causes anger and displeasure in such storylines of the series.