7. Renee Royce and Kelly Severide
What a trainwreck these two were. Renee Royce (Sarah Shahi) was an accomplished international finance lawyer, and she was disappointed that Kelly Severide (Taylor Kinney) wasn't willing to give up everything and move to Madrid with her. When she came back in season 2, however, she was pregnant and claiming that the baby was Severide's.
Royce and Severide move in together, but eventually, it's found out that the baby does not belong to Severide, and the couple, understandably, go their separate ways. It was never in the cards for these two, and the second half of their relationship felt like a trick on Severide, which we don't love to see.