While talking, I realized that one of my characters is a plot killer.

So what is a plot killer? A plot killer is one character that sticks out like a piece for a jigsaw puzzle that's in the wrong box. You just can't make it fit no matter what you do. A plot killer has the best of all intentions and just can't be used for anything good. It's not their fault; it's yours.

I love Cassie, my plot killer. I need her. She's instrumental to the story, but the way I created her made her a detriment to the story. I need her and can't use her.

What do you do with a plot killer? Exterminate them? No. You're the writer, the creator. You change them if you can. Even if that means you're going to rewrite until your fingers fall off and you lose the will to live outside of coffee and Jolly Ranchers. A plot killer needs to be remolded into something useful or they will have to be eliminated.

How did I 'fix' Cassie? I changed her power. Her power was entirely too vague and yet too useless at the same time. I needed her to become useful and thus stripped her power from her and gave her another one. I changed certain aspects of her history and personality to make it fit a bit better. In the process, I removed my plot killer from Awakening and Legacy. Well, one of the plot killers from Legacy anyway.

If you recognize a plot killer in your story, it's going to be okay. You just have to be prepared for a lot of work and make sure that you take detailed notes on your character's new personality. Whether that's new 'powers' or a new backstory, write it down. Also, be honest with yourself. You created this mess, only you can clean it up. Don't be too proud to admit that you have a problem on your hands. The quicker you see the issue, the less clean up you're going to have to do.