5. Caitlin Snow and Marcus
You know how much we were supposed to care about this relationship? The Flash didn't even give Marcus a surname.
Caitlin Snow was one of The Flash's main characters throughout all of its nine seasons, and yet she stopped feeling like one around season 5. The creative team just didn't give her any worthwhile material and, by the time season 8 rolled around, the creativity had hit rock bottom.
Marcus was the first man Caitlin had dated since her back-to-back short-lived romances with Hunter Zolomon and Julian Albert. The woman who had lost her husband Ronnie (twice) and had fallen into relationships with a full-on villain, and an antagonistic semi-villain. The latter relationship came to an end when Killer Frost took over Caitlin's body and arrived with a vengeance.
Did the show happen to acknowledge any of the residual trauma that Caitlin might have experienced over the years because of how it handled her earlier romances? No. Instead, Marcus was introduced as a plot device that could be used to further the tension between Caitlin and her sister Frost. Oh yeah, and he had a similar name to Frost's boyfriend Mark Blaine, a.k.a. Chillblaine.
I think the idea was for Frost to imply that Caitlin's love life wasn't as exciting as her's, but all it pulled off was showing us how far the standard of writing on the show had declined. Anytime a main character is put into a relationship with an unimportant guest character, it's impossible to care for. And fans did not care. At all.