I must say I'm very impressed that you recognized that your OCs in the last story were Mary-Sues, and that you are working to improve your writing. Many writers aren't able to do that.

Since you're trying to improve, I hope you don't mind if I try and help out a bit here by giving you some things to think about:

-Why did Murdoc ask your character to be a choreographer? Don't the Gorillaz have enough money to hire their own prfessional choreographer? If not, why would Murdoc ask your OC? She didn't do anything when she met him to suggest she had talent in that area. She was just hanging out at a bar.

-Who is your OC? We barely know anything about her. What is her past? her personality? Does she have likes and dislikes? Issues she needs to overcome, or strengths she could use to help other characters overcome issues?

-Remember that 2D is a grown man and has been clearly established as a "player" when it comes to women. Remember, he has all those illegitimate children. He probably wouldn't react the way he did in the story to your OC flirting with him. He's not an a-hole like Murdoc, but he's not a shy teenage boy either. He's a playboy rockstar. He can get countless hot women and is probably very casual about bringing them to bed.

-Finally, consider this. Do you need an OC? Look at the Gorillaz section. I'd say over 75% of the stories on the first page alone are stories about a young girl who is a self insert for the author joining Gorillaz and falling in love with 2D or Murdoc. It's been done to death and it's very rarely done well. Consider writing about the Gorillaz characters themselves. Are there stories you could tell from 2D, Russel, Noodle or Murdoc's point of view? About something other than romance? These stories are also fun and are good practice at working on characterization.

These are just some things to think about. Best of luck with your stories!