I've rotated characters frequently, but my first and only drop I'd really consider was Cassie.
Ever since I started MKX, it was my goal to learn and main Cassie. Her personality, hair, outfit, voiceover, it all just screamed a character for me.
Countless upon countless lab hours trying to get the run cancel down after flip kick. And countless upon countless hours for all her other stuff, losing a ridiculous number of games. Then finally, I felt comfortable enough with her to call her my main. It was actually kind of a sense of pride because I never took fighting games seriously and I felt I mastered running mid-combo and Cassie's execution early. Initially I focussed on Brawler, then transitioned to Hollywood cause people said it was better and then finally decided I love Spec Ops. Played in a few online tournaments I found and did decently, and nobody else used the variation, I loved it.
Problem was, I got bored. No one knew the matchup so I could run gimmicks all day and steal games. I never learned a proper neutral, I just learned how to run up, throw out a 50/50 into EX flipkick and setup an overhead/low after. I rarely played defensive and never learned how to because she basically had the touch of death. I felt like I never learned anything playing as her, so I went back to hollywood. By that time though Hollywood was the same, only now I nutpunched instead of a hard-to-blockable and the execution was easier (outside of IAG).
Since then I've bounced around the whole cast, but I think it might be finally time I go back given the robot balls that were just neutered off my flair (funny cause my last comment was how I thought the nerfs weren't bad... well that was when we only knew of 2, not the 18 in the patch notes).