The story I just wrote was interesting. Usually, the characters behave themselves and do what they are supposed to do. In this case, the protagonist’s sidekick kept hogging the limelight. To a certain extent, he was meant to, but he…got carried away. I got carried away with him. I liked the character and he was fun to write, but he was overshadowing the protagonist to the point that I knew readers would question why the love interest didn’t go for the sidekick instead of the protagonist.
Fortunately, I recognized that fairly early in the story and was able to dampen his role and bump up the protagonist so that he “made sense” as the lead. The sidekick didn’t disappear entirely, but he became less present on the page and the protagonist was able to step forward and take his rightful place.
It is easy to fall in love with a character so much that we get carried away in making use of them, but if it isn’t their story, it won’t work. Be sure people are doing what they need to do in order for the story to make sense. If they aren’t, then maybe you aren’t telling the right story.
If X is the hero, let him, even make him, be the hero. It may mean rewriting a lot of what you already have written down, but it is the only way for the story to work. And, if you discover a more interesting story surfaces among the characters, then make the entire story shape around that. Trying to force disparate elements together (character vs. story) just never works. It confuses the reader and doesn’t accomplish your purpose.
Whatever you write, let the best man (or woman, or child, or creature) win.